Method and system for processing luma and chroma signals

ABSTRACT

The present disclosure provides systems and methods for processing video content. The method can include: receiving data representing a first block and a second block in a picture, the data comprising a plurality of chroma samples associated with the first block and a plurality of luma samples associated with the second block; determining an average value of the plurality of luma samples associated with the second block; determining a chroma scaling factor for the first block based on the average value; and processing the plurality of chroma samples associated with the first block using the chroma scaling factor.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. Pat. Application No.17/360,625 filed Jun. 28, 2021, which claims the benefit of priority toU.S. Pat. Application No. 16/887,048, filed on May 29, 2020, whichclaims the benefits of priority to U.S. Provisional Application No.62/865,815, filed Jun. 24, 2019, all of which are incorporated herein byreference in their entireties.

TECHNICAL FIELD

The present disclosure generally relates to video processing, and moreparticularly, to methods and systems for performing luma mapping withchroma scaling.

BACKGROUND

A video is a set of static pictures (or “frames”) capturing the visualinformation. To reduce the storage memory and the transmissionbandwidth, a video can be compressed before storage or transmission anddecompressed before display. The compression process is usually referredto as encoding, and the decompression process is usually referred to asdecoding. There are various video coding formats which use standardizedvideo coding technologies, most commonly based on prediction, transform,quantization, entropy coding and in-loop filtering. The video codingstandards, such as the High Efficiency Video Coding (HEVC/H.265)standard and the Versatile Video Coding (VVC/H.266) standard AVSstandards, specifying the specific video coding formats, are developedby standardization organizations. With more and more advanced videocoding technologies being adopted in the video standards, the codingefficiency of the new video coding standards get higher and higher.

SUMMARY OF THE DISCLOSURE

The embodiments of the present disclosure provide methods and systemsfor performing in-loop luma mapping with chroma scaling and crosscomponent linear model.

In one exemplary embodiment, the method includes: receiving datarepresenting a first block and a second block in a picture, the datacomprising a plurality of chroma samples associated with the first blockand a plurality of luma samples associated with the second block;determining an average value of the plurality of luma samples associatedwith the second block; determining a chroma scaling factor for the firstblock based on the average value; and processing the plurality of chromasamples associated with the first block using the chroma scaling factor.

In some embodiments, the system include: a memory for storing a set ofinstructions; and at least one processor configured to execute the setof instructions to cause the system to perform: receiving datarepresenting a first block and a second block in a picture, the datacomprising a plurality of chroma samples associated with the first blockand a plurality of luma samples associated with the second block;determining an average value of the plurality of luma samples associatedwith the second block; determining a chroma scaling factor for the firstblock based on the average value; and processing the plurality of chromasamples associated with the first block using the chroma scaling factor.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments and various aspects of the present disclosure areillustrated in the following detailed description and the accompanyingfigures. Various features shown in the figures are not drawn to scale.

FIG. 1 illustrates structures of an example video sequence, according tosome embodiments of this disclosure.

FIG. 2A illustrates a schematic diagram of an example encoding process,according to some embodiments of this disclosure.

FIG. 2B illustrates a schematic diagram of another example encodingprocess, according to some embodiments of this disclosure.

FIG. 3A illustrates a schematic diagram of an example decoding process,according to some embodiments of this disclosure.

FIG. 3B illustrates a schematic diagram of another example decodingprocess, according to some embodiments of this disclosure.

FIG. 4 illustrates a block diagram of an example apparatus for encodingor decoding a video, according to some embodiments of this disclosure.

FIG. 5 illustrates a schematic diagram of an exemplary luma mapping withchroma scaling (LMCS) process, according to some embodiments of thedisclosure.

FIG. 6 is a tile group level syntax table for LMCS, according to someembodiments of the disclosure.

FIG. 7 is a syntax table for LMCS, according to some embodiments of thedisclosure.

FIG. 8 is a slice level syntax table for LMCS, according to someembodiments of the disclosure.

FIG. 9 is a syntax table for LMCS piecewise linear model, according tosome embodiments of the disclosure.

FIG. 10 illustrates an example of locations of samples used forderivation of α and β, according to some embodiments of the disclosure.

FIG. 11 is a table for derivation of chroma prediction mode from lumamode when CCLM is enabled, according to some embodiments of thedisclosure

FIG. 12 is an exemplary coding tree unit syntax structure, according tosome embodiments of the disclosure.

FIG. 13 is an exemplary dual tree partition syntax structure, accordingto some embodiments of the disclosure.

FIG. 14 is an exemplary coding tree unit syntax structure, according tosome embodiments of the disclosure.

FIG. 15 is an exemplary dual tree partition syntax structure, accordingto some embodiments of the disclosure.

FIG. 16A illustrates an exemplary chroma tree partition, according tosome embodiments of the disclosure.

FIG. 16B illustrates an exemplary luma tree partition, according to someembodiments of the disclosure.

FIG. 17 illustrates an exemplary simplification of an averagingoperation, according to some embodiments of the disclosure.

FIG. 18 illustrates examples of samples used in average calculation toderive a chroma scaling factor, according to some embodiments of thedisclosure.

FIG. 19 illustrates an example of chroma scaling factor derivation forblocks at picture right or bottom boundary, according to someembodiments of the disclosure.

FIG. 20 is an exemplary coding tree unit syntax structure, according tosome embodiments of the disclosure.

FIG. 21 is another exemplary coding tree unit syntax structure,according to some embodiments of the disclosure.

FIG. 22 is an exemplary modified signaling of the LMCS piecewise linearmodel at slice level, according to some embodiments of the disclosure.

FIG. 23 is a flowchart of an exemplary method for processing videocontent, according to some embodiments of the disclosure.

DETAILED DESCRIPTION

Reference will now be made in detail to exemplary embodiments, examplesof which are illustrated in the accompanying drawings. The followingdescription refers to the accompanying drawings in which the samenumbers in different drawings represent the same or similar elementsunless otherwise represented. The implementations set forth in thefollowing description of exemplary embodiments do not represent allimplementations consistent with the invention. Instead, they are merelyexamples of apparatuses and methods consistent with aspects related tothe invention as recited in the appended claims. Unless specificallystated otherwise, the term “or” encompasses all possible combinations,except where infeasible. For example, if it is stated that a componentmay include A or B, then, unless specifically stated otherwise orinfeasible, the component may include A, or B, or A and B. As a secondexample, if it is stated that a component may include A, B, or C, then,unless specifically stated otherwise or infeasible, the component mayinclude A, or B, or C, or A and B, or A and C, or B and C, or A and Band C.

A video is a set of static pictures (or “frames”) arranged in a temporalsequence to store visual information. A video capture device (e.g., acamera) can be used to capture and store those pictures in a temporalsequence, and a video playback device (e.g., a television, a computer, asmartphone, a tablet computer, a video player, or any end-user terminalwith a function of display) can be used to display such pictures in thetemporal sequence. Also, in some applications, a video capturing devicecan transmit the captured video to the video playback device (e.g., acomputer with a monitor) in real-time, such as for monitoring,conferencing, or live broadcasting.

For reducing the storage space and the transmission bandwidth needed bysuch applications, the video can be compressed before storage andtransmission and decompressed before the display. The compression anddecompression can be implemented by software executed by a processor(e.g., a processor of a generic computer) or specialized hardware. Themodule for compression is generally referred to as an “encoder,” and themodule for decompression is generally referred to as a “decoder.” Theencoder and decoder can be collectively referred to as a “codec.” Theencoder and decoder can be implemented as any of a variety of suitablehardware, software, or a combination thereof. For example, the hardwareimplementation of the encoder and decoder can include circuitry, such asone or more microprocessors, digital signal processors (DSPs),application-specific integrated circuits (ASICs), field-programmablegate arrays (FPGAs), discrete logic, or any combinations thereof. Thesoftware implementation of the encoder and decoder can include programcodes, computer-executable instructions, firmware, or any suitablecomputer-implemented algorithm or process fixed in a computer-readablemedium. Video compression and decompression can be implemented byvarious algorithms or standards, such as MPEG- 1, MPEG-2, MPEG-4, H.26xseries, or the like. In some applications, the codec can decompress thevideo from a first coding standard and re-compress the decompressedvideo using a second coding standard, in which case the codec can bereferred to as a “transcoder.”

The video encoding process can identify and keep useful information thatcan be used to reconstruct a picture and disregard unimportantinformation for the reconstruction. If the disregarded, unimportantinformation cannot be fully reconstructed, such an encoding process canbe referred to as “lossy.” Otherwise, it can be referred to as“lossless.” Most encoding processes are lossy, which is a tradeoff toreduce the needed storage space and the transmission bandwidth.

The useful information of a picture being encoded (referred to as a“current picture”) include changes with respect to a reference picture(e.g., a picture previously encoded and reconstructed). Such changes caninclude position changes, luminosity changes, or color changes of thepixels, among which the position changes are mostly concerned. Positionchanges of a group of pixels that represent an object can reflect themotion of the object between the reference picture and the currentpicture.

A picture coded without referencing another picture (i.e., it is its ownreference picture) is referred to as an “I-picture.” A picture codedusing a previous picture as a reference picture is referred to as a“P-picture.” A picture coded using both a previous picture and a futurepicture as reference pictures (i.e., the reference is “bi-directional”)is referred to as a “B-picture.”

As previously mentioned, one of the goals for developing new videocoding techniques is to improve the coding efficiency, i.e., using lesscoded data to represent the same picture quality. The present disclosureprovides methods and systems for performing luma mapping with chromascaling. The luma mapping is a process for mapping luma samples to usein the loop filter, and the chroma scaling is a luma-dependent processfor scaling chroma residue values. ve the same subjective quality asHEVC/H.265 using half the bandwidth LMCS has two main components: 1) aprocess for mapping input luma code values to a new set of code valuesfor use inside the coding loop; and 2) a luma-dependent process forscaling chroma residue values. The luma mapping process improves thecoding efficiency for standard and high dynamic range video signals bymaking better use of the range of luma code values allowed at aspecified bit depth.

FIG. 1 illustrates structures of an example video sequence 100 using invideo coding, according to some embodiments of this disclosure. Videosequence 100 can be a live video or a video having been captured andarchived. Video 100 can be a real-life video, a computer-generated video(e.g., computer game video), or a combination thereof (e.g., a real-lifevideo with augmented-reality effects). Video sequence 100 can beinputted from a video capture device (e.g., a camera), a video archive(e.g., a video file stored in a storage device) containing previouslycaptured video, or a video feed interface (e.g., a video broadcasttransceiver) to receive video from a video content provider.

As shown in FIG. 1 , video sequence 100 can include a series of picturesarranged temporally along a timeline, including pictures 102, 104, 106,and 108. Pictures 102-106 are continuous, and there are more picturesbetween pictures 106 and 108. In FIG. 1 , picture 102 is an I-picture,the reference picture of which is picture 102 itself. Picture 104 is aP-picture, the reference picture of which is picture 102, as indicatedby the arrow. Picture 106 is a B-picture, the reference pictures ofwhich are pictures 104 and 108, as indicated by the arrows. In someembodiments, the reference picture of a picture (e.g., picture 104) canbe not immediately preceding or following the picture. For example, thereference picture of picture 104 can be a picture preceding picture 102.It should be noted that the reference pictures of pictures 102-106 areonly examples, and this disclosure does not limit embodiments of thereference pictures as the examples shown in FIG. 1 .

Typically, video codecs do not encode or decode an entire picture at onetime due to the computing complexity of such tasks. Rather, they cansplit the picture into basic segments, and encode or decode the picturesegment by segment. Such basic segments are referred to as basicprocessing units (“BPUs”) in this disclosure. For example, structure 110in FIG. 1 shows an example structure of a picture of video sequence 100(e.g., any of pictures 102-108). In structure 110, a picture is dividedinto 4×4 basic processing units, the boundaries of which are shown asdash lines. In some embodiments, the basic processing units can bereferred to as “macroblocks” in some video coding standards (e.g., MPEGfamily, H.261, H.263, or H.264/AVC), or as “coding tree units” (“CTUs”)in some other video coding standards (e.g., H.265/HEVC or H.266/VVC).The basic processing units can have variable sizes in a picture, such as128×128, 64×64, 32×32, 16×16, 4×8, 16×32, or any arbitrary shape andsize of pixels. The sizes and shapes of the basic processing units canbe selected for a picture based on the balance of coding efficiency andlevels of details to be kept in the basic processing unit.

The basic processing units can be logical units, which can include agroup of different types of video data stored in a computer memory(e.g., in a video frame buffer). For example, a basic processing unit ofa color picture can include a luma component (Y) representing achromaticbrightness information, one or more chroma components (e.g., Cb and Cr)representing color information, and associated syntax elements, in whichthe luma and chroma components can have the same size of the basicprocessing unit. The luma and chroma components can be referred to as“coding tree blocks” (“CTB s”) in some video coding standards (e.g.,H.265/HEVC or H.266/VVC). Any operation performed to a basic processingunit can be repeatedly performed to each of its luma and chromacomponents.

Video coding has multiple stages of operations, examples of which willbe detailed in FIGS. 2A-2B and 3A-3B. For each stage, the size of thebasic processing units can still be too large for processing, and thuscan be further divided into segments referred to as “basic processingsub-units” in this disclosure. In some embodiments, the basic processingsub-units can be referred to as “blocks” in some video coding standards(e.g., MPEG family, H.261, H.263, or H.264/AVC), or as “coding units”(“CUs”) in some other video coding standards (e.g., H.265/HEVC orH.266/VVC). A basic processing sub-unit can have the same or smallersize than the basic processing unit. Similar to the basic processingunits, basic processing sub-units are also logical units, which caninclude a group of different types of video data (e.g., Y, Cb, Cr, andassociated syntax elements) stored in a computer memory (e.g., in avideo frame buffer). Any operation performed to a basic processingsub-unit can be repeatedly performed to each of its luma and chromacomponents. It should be noted that such division can be performed tofurther levels depending on processing needs. It should also be notedthat different stages can divide the basic processing units usingdifferent schemes.

For example, at a mode decision stage (an example of which will bedetailed in FIG. 2B), the encoder can decide what prediction mode (e.g.,intra-picture prediction or inter-picture prediction) to use for a basicprocessing unit, which can be too large to make such a decision. Theencoder can split the basic processing unit into multiple basicprocessing sub-units (e.g., CUs as in H.265/HEVC or H.266/VVC), anddecide a prediction type for each individual basic processing sub-unit.

For another example, at a prediction stage (an example of which will bedetailed in FIG. 2A), the encoder can perform prediction operation atthe level of basic processing sub-units (e.g., CUs). However, in somecases, a basic processing sub-unit can still be too large to process.The encoder can further split the basic processing sub-unit into smallersegments (e.g., referred to as “prediction blocks” or “PBs” inH.265/HEVC or H.266/VVC), at the level of which the prediction operationcan be performed.

For another example, at a transform stage (an example of which will bedetailed in FIG. 2A), the encoder can perform a transform operation forresidual basic processing sub-units (e.g., CUs). However, in some cases,a basic processing sub-unit can still be too large to process. Theencoder can further split the basic processing sub-unit into smallersegments (e.g., referred to as “transform blocks” or “TBs” in H.265/HEVCor H.266/VVC), at the level of which the transform operation can beperformed. It should be noted that the division schemes of the samebasic processing sub-unit can be different at the prediction stage andthe transform stage. For example, in H.265/HEVC or H.266/VVC, theprediction blocks and transform blocks of the same CU can have differentsizes and numbers.

In structure 110 of FIG. 1 , basic processing unit 112 is furtherdivided into 3×3 basic processing sub-units, the boundaries of which areshown as dotted lines. Different basic processing units of the samepicture can be divided into basic processing sub-units in differentschemes.

In some implementations, to provide the capability of parallelprocessing and error resilience to video encoding and decoding, apicture can be divided into regions for processing, such that, for aregion of the picture, the encoding or decoding process can depend on noinformation from any other region of the picture. In other words, eachregion of the picture can be processed independently. By doing so, thecodec can process different regions of a picture in parallel, thusincreasing the coding efficiency. Also, when data of a region iscorrupted in the processing or lost in network transmission, the codeccan correctly encode or decode other regions of the same picture withoutreliance on the corrupted or lost data, thus providing the capability oferror resilience. In some video coding standards, a picture can bedivided into different types of regions. For example, H.265/HEVC andH.266/VVC provide two types of regions: “slices” and “tiles.” It shouldalso be noted that different pictures of video sequence 100 can havedifferent partition schemes for dividing a picture into regions.

For example, in FIG. 1 , structure 110 is divided into three regions114, 116, and 118, the boundaries of which are shown as solid linesinside structure 110. Region 114 includes four basic processing units.Each of regions 116 and 118 includes six basic processing units. Itshould be noted that the basic processing units, basic processingsub-units, and regions of structure 110 in FIG. 1 are only examples, andthis disclosure does not limit embodiments thereof.

FIG. 2A illustrates a schematic diagram of an example encoding process200A, according to some embodiments of this disclosure. An encoder canencode video sequence 202 into video bitstream 228 according to process200A. Similar to video sequence 100 in FIG. 1 , video sequence 202 caninclude a set of pictures (referred to as “original pictures”) arrangedin a temporal order. Similar to structure 110 in FIG. 1 , each originalpicture of video sequence 202 can be divided by the encoder into basicprocessing units, basic processing sub-units, or regions for processing.In some embodiments, the encoder can perform process 200A at the levelof basic processing units for each original picture of video sequence202. For example, the encoder can perform process 200A in an iterativemanner, in which the encoder can encode a basic processing unit in oneiteration of process 200A. In some embodiments, the encoder can performprocess 200A in parallel for regions (e.g., regions 114-118) of eachoriginal picture of video sequence 202.

In FIG. 2A, the encoder can feed a basic processing unit (referred to asan “original BPU”) of an original picture of video sequence 202 toprediction stage 204 to generate prediction data 206 and predicted BPU208. The encoder can subtract predicted BPU 208 from the original BPU togenerate residual BPU 210. The encoder can feed residual BPU 210 totransform stage 212 and quantization stage 214 to generate quantizedtransform coefficients 216. The encoder can feed prediction data 206 andquantized transform coefficients 216 to binary coding stage 226 togenerate video bitstream 228. Components 202, 204, 206, 208, 210, 212,214, 216, 226, and 228 can be referred to as a “forward path.” Duringprocess 200A, after quantization stage 214, the encoder can feedquantized transform coefficients 216 to inverse quantization stage 218and inverse transform stage 220 to generate reconstructed residual BPU222. The encoder can add reconstructed residual BPU 222 to predicted BPU208 to generate prediction reference 224, which is used in predictionstage 204 for the next iteration of process 200A. Components 218, 220,222, and 224 of process 200A can be referred to as a “reconstructionpath.” The reconstruction path can be used to ensure that both theencoder and the decoder use the same reference data for prediction.

The encoder can perform process 200A iteratively to encode each originalBPU of the original picture (in the forward path) and generate predictedreference 224 for encoding the next original BPU of the original picture(in the reconstruction path). After encoding all original BPUs of theoriginal picture, the encoder can proceed to encode the next picture invideo sequence 202.

Referring to process 200A, the encoder can receive video sequence 202generated by a video capturing device (e.g., a camera). The term“receive” used herein can refer to receiving, inputting, acquiring,retrieving, obtaining, reading, accessing, or any action in any mannerfor inputting data.

At prediction stage 204, at a current iteration, the encoder can receivean original BPU and prediction reference 224, and perform a predictionoperation to generate prediction data 206 and predicted BPU 208.Prediction reference 224 can be generated from the reconstruction pathof the previous iteration of process 200A. The purpose of predictionstage 204 is to reduce information redundancy by extracting predictiondata 206 that can be used to reconstruct the original BPU as predictedBPU 208 from prediction data 206 and prediction reference 224.

Ideally, predicted BPU 208 can be identical to the original BPU.However, due to non-ideal prediction and reconstruction operations,predicted BPU 208 is generally slightly different from the original BPU.For recording such differences, after generating predicted BPU 208, theencoder can subtract it from the original BPU to generate residual BPU210. For example, the encoder can subtract values (e.g., greyscalevalues or RGB values) of pixels of predicted BPU 208 from values ofcorresponding pixels of the original BPU. Each pixel of residual BPU 210can have a residual value as a result of such subtraction between thecorresponding pixels of the original BPU and predicted BPU 208. Comparedwith the original BPU, prediction data 206 and residual BPU 210 can havefewer bits, but they can be used to reconstruct the original BPU withoutsignificant quality deterioration. Thus, the original BPU is compressed.

To further compress residual BPU 210, at transform stage 212, theencoder can reduce spatial redundancy of residual BPU 210 by decomposingit into a set of two-dimensional “base patterns,” each base patternbeing associated with a “transform coefficient.” The base patterns canhave the same size (e.g., the size of residual BPU 210). Each basepattern can represent a variation frequency (e.g., frequency ofbrightness variation) component of residual BPU 210. None of the basepatterns can be reproduced from any combinations (e.g., linearcombinations) of any other base patterns. In other words, thedecomposition can decompose variations of residual BPU 210 into afrequency domain. Such a decomposition is analogous to a discreteFourier transform of a function, in which the base patterns areanalogous to the base functions (e.g., trigonometry functions) of thediscrete Fourier transform, and the transform coefficients are analogousto the coefficients associated with the base functions.

Different transform algorithms can use different base patterns. Varioustransform algorithms can be used at transform stage 212, such as, forexample, a discrete cosine transform, a discrete sine transform, or thelike. The transform at transform stage 212 is invertible. That is, theencoder can restore residual BPU 210 by an inverse operation of thetransform (referred to as an “inverse transform”). For example, torestore a pixel of residual BPU 210, the inverse transform can bemultiplying values of corresponding pixels of the base patterns byrespective associated coefficients and adding the products to produce aweighted sum. For a video coding standard, both the encoder and decodercan use the same transform algorithm (thus the same base patterns).Thus, the encoder can record only the transform coefficients, from whichthe decoder can reconstruct residual BPU 210 without receiving the basepatterns from the encoder. Compared with residual BPU 210, the transformcoefficients can have fewer bits, but they can be used to reconstructresidual BPU 210 without significant quality deterioration. Thus,residual BPU 210 is further compressed.

The encoder can further compress the transform coefficients atquantization stage 214. In the transform process, different basepatterns can represent different variation frequencies (e.g., brightnessvariation frequencies). Because human eyes are generally better atrecognizing low-frequency variation, the encoder can disregardinformation of high-frequency variation without causing significantquality deterioration in decoding. For example, at quantization stage214, the encoder can generate quantized transform coefficients 216 bydividing each transform coefficient by an integer value (referred to asa “quantization parameter”) and rounding the quotient to its nearestinteger. After such an operation, some transform coefficients of thehigh-frequency base patterns can be converted to zero, and the transformcoefficients of the low-frequency base patterns can be converted tosmaller integers. The encoder can disregard the zero-value quantizedtransform coefficients 216, by which the transform coefficients arefurther compressed. The quantization process is also invertible, inwhich quantized transform coefficients 216 can be reconstructed to thetransform coefficients in an inverse operation of the quantization(referred to as “inverse quantization”).

Because the encoder disregards the remainders of such divisions in therounding operation, quantization stage 214 can be lossy. Typically,quantization stage 214 can contribute the most information loss inprocess 200A. The larger the information loss is, the fewer bits thequantized transform coefficients 216 can need. For obtaining differentlevels of information loss, the encoder can use different values of thequantization parameter or any other parameter of the quantizationprocess.

At binary coding stage 226, the encoder can encode prediction data 206and quantized transform coefficients 216 using a binary codingtechnique, such as, for example, entropy coding, variable length coding,arithmetic coding, Huffman coding, context-adaptive binary arithmeticcoding, or any other lossless or lossy compression algorithm. In someembodiments, besides prediction data 206 and quantized transformcoefficients 216, the encoder can encode other information at binarycoding stage 226, such as, for example, a prediction mode used atprediction stage 204, parameters of the prediction operation, atransform type at transform stage 212, parameters of the quantizationprocess (e.g., quantization parameters), an encoder control parameter(e.g., a bitrate control parameter), or the like. The encoder can usethe output data of binary coding stage 226 to generate video bitstream228. In some embodiments, video bitstream 228 can be further packetizedfor network transmission.

Referring to the reconstruction path of process 200A, at inversequantization stage 218, the encoder can perform inverse quantization onquantized transform coefficients 216 to generate reconstructed transformcoefficients. At inverse transform stage 220, the encoder can generatereconstructed residual BPU 222 based on the reconstructed transformcoefficients. The encoder can add reconstructed residual BPU 222 topredicted BPU 208 to generate prediction reference 224 that is to beused in the next iteration of process 200A.

It should be noted that other variations of the process 200A can be usedto encode video sequence 202. In some embodiments, stages of process200A can be performed by the encoder in different orders. In someembodiments, one or more stages of process 200A can be combined into asingle stage. In some embodiments, a single stage of process 200A can bedivided into multiple stages. For example, transform stage 212 andquantization stage 214 can be combined into a single stage. In someembodiments, process 200A can include additional stages. In someembodiments, process 200A can omit one or more stages in FIG. 2A.

FIG. 2B illustrates a schematic diagram of another example encodingprocess 200B, according to some embodiments of this disclosure. Process200B can be modified from process 200A. For example, process 200B can beused by an encoder conforming to a hybrid video coding standard (e.g.,H.26x series). Compared with process 200A, the forward path of process200B additionally includes mode decision stage 230 and dividesprediction stage 204 into spatial prediction stage 2042 and temporalprediction stage 2044. The reconstruction path of process 200Badditionally includes loop filter stage 232 and buffer 234.

Generally, prediction techniques can be categorized into two types:spatial prediction and temporal prediction. Spatial prediction (e.g., anintra-picture prediction or “intra prediction”) can use pixels from oneor more already coded neighboring BPUs in the same picture to predictthe current BPU. That is, prediction reference 224 in the spatialprediction can include the neighboring BPUs. The spatial prediction canreduce the inherent spatial redundancy of the picture. Temporalprediction (e.g., an inter-picture prediction or “inter prediction”) canuse regions from one or more already coded pictures to predict thecurrent BPU. That is, prediction reference 224 in the temporalprediction can include the coded pictures. The temporal prediction canreduce the inherent temporal redundancy of the pictures.

Referring to process 200B, in the forward path, the encoder performs theprediction operation at spatial prediction stage 2042 and temporalprediction stage 2044. For example, at spatial prediction stage 2042,the encoder can perform the intra prediction. For an original BPU of apicture being encoded, prediction reference 224 can include one or moreneighboring BPUs that have been encoded (in the forward path) andreconstructed (in the reconstructed path) in the same picture. Theencoder can generate predicted BPU 208 by extrapolating the neighboringBPUs. The extrapolation technique can include, for example, a linearextrapolation or interpolation, a polynomial extrapolation orinterpolation, or the like. In some embodiments, the encoder can performthe extrapolation at the pixel level, such as by extrapolating values ofcorresponding pixels for each pixel of predicted BPU 208. Theneighboring BPUs used for extrapolation can be located with respect tothe original BPU from various directions, such as in a verticaldirection (e.g., on top of the original BPU), a horizontal direction(e.g., to the left of the original BPU), a diagonal direction (e.g., tothe down-left, downright, up-left, or up-right of the original BPU), orany direction defined in the used video coding standard. For the intraprediction, prediction data 206 can include, for example, locations(e.g., coordinates) of the used neighboring BPUs, sizes of the usedneighboring BPUs, parameters of the extrapolation, a direction of theused neighboring BPUs with respect to the original BPU, or the like.

For another example, at temporal prediction stage 2044, the encoder canperform the inter prediction. For an original BPU of a current picture,prediction reference 224 can include one or more pictures (referred toas “reference pictures”) that have been encoded (in the forward path)and reconstructed (in the reconstructed path). In some embodiments, areference picture can be encoded and reconstructed BPU by BPU. Forexample, the encoder can add reconstructed residual BPU 222 to predictedBPU 208 to generate a reconstructed BPU. When all reconstructed BPUs ofthe same picture are generated, the encoder can generate a reconstructedpicture as a reference picture. The encoder can perform an operation of“motion estimation” to search for a matching region in a scope (referredto as a “search window”) of the reference picture. The location of thesearch window in the reference picture can be determined based on thelocation of the original BPU in the current picture. For example, thesearch window can be centered at a location having the same coordinatesin the reference picture as the original BPU in the current picture andcan be extended out for a predetermined distance. When the encoderidentifies (e.g., by using a pel-recursive algorithm, a block-matchingalgorithm, or the like) a region similar to the original BPU in thesearch window, the encoder can determine such a region as the matchingregion. The matching region can have different dimensions (e.g., beingsmaller than, equal to, larger than, or in a different shape) from theoriginal BPU. Because the reference picture and the current picture aretemporally separated in the timeline (e.g., as shown in FIG. 1 ), it canbe deemed that the matching region “moves” to the location of theoriginal BPU as time goes by. The encoder can record the direction anddistance of such a motion as a “motion vector.” When multiple referencepictures are used (e.g., as picture 106 in FIG. 1 ), the encoder cansearch for a matching region and determine its associated motion vectorfor each reference picture. In some embodiments, the encoder can assignweights to pixel values of the matching regions of respective matchingreference pictures.

The motion estimation can be used to identify various types of motions,such as, for example, translations, rotations, zooming, or the like. Forinter prediction, prediction data 206 can include, for example,locations (e.g., coordinates) of the matching region, the motion vectorsassociated with the matching region, the number of reference pictures,weights associated with the reference pictures, or the like.

For generating predicted BPU 208, the encoder can perform an operationof “motion compensation.” The motion compensation can be used toreconstruct predicted BPU 208 based on prediction data 206 (e.g., themotion vector) and prediction reference 224. For example, the encodercan move the matching region of the reference picture according to themotion vector, in which the encoder can predict the original BPU of thecurrent picture. When multiple reference pictures are used (e.g., aspicture 106 in FIG. 1 ), the encoder can move the matching regions ofthe reference pictures according to the respective motion vectors andaverage pixel values of the matching regions. In some embodiments, ifthe encoder has assigned weights to pixel values of the matching regionsof respective matching reference pictures, the encoder can add aweighted sum of the pixel values of the moved matching regions.

In some embodiments, the inter prediction can be unidirectional orbidirectional. Unidirectional inter predictions can use one or morereference pictures in the same temporal direction with respect to thecurrent picture. For example, picture 104 in FIG. 1 is a unidirectionalinter-predicted picture, in which the reference picture (i.e., picture102) precedes picture 104. Bidirectional inter predictions can use oneor more reference pictures at both temporal directions with respect tothe current picture. For example, picture 106 in FIG. 1 is abidirectional inter-predicted picture, in which the reference pictures(i.e., pictures 104 and 108) are at both temporal directions withrespect to picture 104.

Still referring to the forward path of process 200B, after spatialprediction 2042 and temporal prediction stage 2044, at mode decisionstage 230, the encoder can select a prediction mode (e.g., one of theintra prediction or the inter prediction) for the current iteration ofprocess 200B. For example, the encoder can perform a rate-distortionoptimization technique, in which the encoder can select a predictionmode to minimize a value of a cost function depending on a bit rate of acandidate prediction mode and distortion of the reconstructed referencepicture under the candidate prediction mode. Depending on the selectedprediction mode, the encoder can generate the corresponding predictedBPU 208 and predicted data 206.

In the reconstruction path of process 200B, if intra prediction mode hasbeen selected in the forward path, after generating prediction reference224 (e.g., the current BPU that has been encoded and reconstructed inthe current picture), the encoder can directly feed prediction reference224 to spatial prediction stage 2042 for later usage (e.g., forextrapolation of a next BPU of the current picture). If the interprediction mode has been selected in the forward path, after generatingprediction reference 224 (e.g., the current picture in which all BPUshave been encoded and reconstructed), the encoder can feed predictionreference 224 to loop filter stage 232, at which the encoder can apply aloop filter to prediction reference 224 to reduce or eliminatedistortion (e.g., blocking artifacts) introduced by the interprediction. The encoder can apply various loop filter techniques at loopfilter stage 232, such as, for example, deblocking, sample adaptiveoffsets, adaptive loop filters, or the like. The loop-filtered referencepicture can be stored in buffer 234 (or “decoded picture buffer”) forlater use (e.g., to be used as an inter-prediction reference picture fora future picture of video sequence 202). The encoder can store one ormore reference pictures in buffer 234 to be used at temporal predictionstage 2044. In some embodiments, the encoder can encode parameters ofthe loop filter (e.g., a loop filter strength) at binary coding stage226, along with quantized transform coefficients 216, prediction data206, and other information.

FIG. 3A illustrates a schematic diagram of an example decoding process300A, according to some embodiments of this disclosure. Process 300A canbe a decompression process corresponding to the compression process 200Ain FIG. 2A. In some embodiments, process 300A can be similar to thereconstruction path of process 200A. A decoder can decode videobitstream 228 into video stream 304 according to process 300A. Videostream 304 can be very similar to video sequence 202. However, due tothe information loss in the compression and decompression process (e.g.,quantization stage 214 in FIGS. 2A-2B), generally, video stream 304 isnot identical to video sequence 202. Similar to processes 200A and 200Bin FIGS. 2A-2B, the decoder can perform process 300A at the level ofbasic processing units (BPUs) for each picture encoded in videobitstream 228. For example, the decoder can perform process 300A in aniterative manner, in which the decoder can decode a basic processingunit in one iteration of process 300A. In some embodiments, the decodercan perform process 300A in parallel for regions (e.g., regions 114-118)of each picture encoded in video bitstream 228.

In FIG. 3A, the decoder can feed a portion of video bitstream 228associated with a basic processing unit (referred to as an “encodedBPU”) of an encoded picture to binary decoding stage 302. At binarydecoding stage 302, the decoder can decode the portion into predictiondata 206 and quantized transform coefficients 216. The decoder can feedquantized transform coefficients 216 to inverse quantization stage 218and inverse transform stage 220 to generate reconstructed residual BPU222. The decoder can feed prediction data 206 to prediction stage 204 togenerate predicted BPU 208. The decoder can add reconstructed residualBPU 222 to predicted BPU 208 to generate predicted reference 224. Insome embodiments, predicted reference 224 can be stored in a buffer(e.g., a decoded picture buffer in a computer memory). The decoder canfeed predicted reference 224 to prediction stage 204 for performing aprediction operation in the next iteration of process 300A.

The decoder can perform process 300A iteratively to decode each encodedBPU of the encoded picture and generate predicted reference 224 forencoding the next encoded BPU of the encoded picture. After decoding allencoded BPUs of the encoded picture, the decoder can output the pictureto video stream 304 for display and proceed to decode the next encodedpicture in video bitstream 228.

At binary decoding stage 302, the decoder can perform an inverseoperation of the binary coding technique used by the encoder (e.g.,entropy coding, variable length coding, arithmetic coding, Huffmancoding, context-adaptive binary arithmetic coding, or any other losslesscompression algorithm). In some embodiments, besides prediction data 206and quantized transform coefficients 216, the decoder can decode otherinformation at binary decoding stage 302, such as, for example, aprediction mode, parameters of the prediction operation, a transformtype, parameters of the quantization process (e.g., quantizationparameters), an encoder control parameter (e.g., a bitrate controlparameter), or the like. In some embodiments, if video bitstream 228 istransmitted over a network in packets, the decoder can depacketize videobitstream 228 before feeding it to binary decoding stage 302.

FIG. 3B illustrates a schematic diagram of another example decodingprocess 300B, according to some embodiments of this disclosure. Process300B can be modified from process 300A. For example, process 300B can beused by a decoder conforming to a hybrid video coding standard (e.g.,H.26x series). Compared with process 300A, process 300B additionallydivides prediction stage 204 into spatial prediction stage 2042 andtemporal prediction stage 2044, and additionally includes loop filterstage 232 and buffer 234.

In process 300B, for an encoded basic processing unit (referred to as a“current BPU”) of an encoded picture (referred to as a “currentpicture”) that is being decoded, prediction data 206 decoded from binarydecoding stage 302 by the decoder can include various types of data,depending on what prediction mode was used to encode the current BPU bythe encoder. For example, if intra prediction was used by the encoder toencode the current BPU, prediction data 206 can include a predictionmode indicator (e.g., a flag value) indicative of the intra prediction,parameters of the intra prediction operation, or the like. Theparameters of the intra prediction operation can include, for example,locations (e.g., coordinates) of one or more neighboring BPUs used as areference, sizes of the neighboring BPUs, parameters of extrapolation, adirection of the neighboring BPUs with respect to the original BPU, orthe like. For another example, if inter prediction was used by theencoder to encode the current BPU, prediction data 206 can include aprediction mode indicator (e.g., a flag value) indicative of the interprediction, parameters of the inter prediction operation, or the like.The parameters of the inter prediction operation can include, forexample, the number of reference pictures associated with the currentBPU, weights respectively associated with the reference pictures,locations (e.g., coordinates) of one or more matching regions in therespective reference pictures, one or more motion vectors respectivelyassociated with the matching regions, or the like.

Based on the prediction mode indicator, the decoder can decide whetherto perform a spatial prediction (e.g., the intra prediction) at spatialprediction stage 2042 or a temporal prediction (e.g., the interprediction) at temporal prediction stage 2044. The details of performingsuch spatial prediction or temporal prediction are described in FIG. 2Band will not be repeated hereinafter. After performing such spatialprediction or temporal prediction, the decoder can generate predictedBPU 208. The decoder can add predicted BPU 208 and reconstructedresidual BPU 222 to generate prediction reference 224, as described inFIG. 3A.

In process 300B, the decoder can feed predicted reference 224 to spatialprediction stage 2042 or temporal prediction stage 2044 for performing aprediction operation in the next iteration of process 300B. For example,if the current BPU is decoded using the intra prediction at spatialprediction stage 2042, after generating prediction reference 224 (e.g.,the decoded current BPU), the decoder can directly feed predictionreference 224 to spatial prediction stage 2042 for later usage (e.g.,for extrapolation of a next BPU of the current picture). If the currentBPU is decoded using the inter prediction at temporal prediction stage2044, after generating prediction reference 224 (e.g., a referencepicture in which all BPUs have been decoded), the encoder can feedprediction reference 224 to loop filter stage 232 to reduce or eliminatedistortion (e.g., blocking artifacts). The decoder can apply a loopfilter to prediction reference 224, in a way as described in FIG. 2B.The loop-filtered reference picture can be stored in buffer 234 (e.g., adecoded picture buffer in a computer memory) for later use (e.g., to beused as an inter-prediction reference picture for a future encodedpicture of video bitstream 228). The decoder can store one or morereference pictures in buffer 234 to be used at temporal prediction stage2044. In some embodiments, when the prediction mode indicator ofprediction data 206 indicates that inter prediction was used to encodethe current BPU, prediction data can further include parameters of theloop filter (e.g., a loop filter strength).

FIG. 4 is a block diagram of an example apparatus 400 for encoding ordecoding a video, according to some embodiments of this disclosure. Asshown in FIG. 4 , apparatus 400 can include processor 402. Whenprocessor 402 executes instructions described herein, apparatus 400 canbecome a specialized machine for video encoding or decoding. Processor402 can be any type of circuitry capable of manipulating or processinginformation. For example, processor 402 can include any combination ofany number of a central processing unit (or “CPU”), a graphicsprocessing unit (or “GPU”), a neural processing unit (“NPU”), amicrocontroller unit (“MCU”), an optical processor, a programmable logiccontroller, a microcontroller, a microprocessor, a digital signalprocessor, an intellectual property (IP) core, a Programmable LogicArray (PLA), a Programmable Array Logic (PAL), a Generic Array Logic(GAL), a Complex Programmable Logic Device (CPLD), a Field-ProgrammableGate Array (FPGA), a System On Chip (SoC), an Application-SpecificIntegrated Circuit (ASIC), or the like. In some embodiments, processor402 can also be a set of processors grouped as a single logicalcomponent. For example, as shown in FIG. 4 , processor 402 can includemultiple processors, including processor 402 a, processor 402 b, andprocessor 402 n.

Apparatus 400 can also include memory 404 configured to store data(e.g., a set of instructions, computer codes, intermediate data, or thelike). For example, as shown in FIG. 4 , the stored data can includeprogram instructions (e.g., program instructions for implementing thestages in processes 200A, 200B, 300A, or 300B) and data for processing(e.g., video sequence 202, video bitstream 228, or video stream 304).Processor 402 can access the program instructions and data forprocessing (e.g., via bus 410), and execute the program instructions toperform an operation or manipulation on the data for processing. Memory404 can include a high-speed random-access storage device or anon-volatile storage device. In some embodiments, memory 404 can includeany combination of any number of a random-access memory (RAM), aread-only memory (ROM), an optical disc, a magnetic disk, a hard drive,a solid-state drive, a flash drive, a security digital (SD) card, amemory stick, a compact flash (CF) card, or the like. Memory 404 canalso be a group of memories (not shown in FIG. 4 ) grouped as a singlelogical component.

Bus 410 can be a communication device that transfers data betweencomponents inside apparatus 400, such as an internal bus (e.g., aCPU-memory bus), an external bus (e.g., a universal serial bus port, aperipheral component interconnect express port), or the like.

For ease of explanation without causing ambiguity, processor 402 andother data processing circuits are collectively referred to as a “dataprocessing circuit” in this disclosure. The data processing circuit canbe implemented entirely as hardware, or as a combination of software,hardware, or firmware. In addition, the data processing circuit can be asingle independent module or can be combined entirely or partially intoany other component of apparatus 400.

Apparatus 400 can further include network interface 406 to provide wiredor wireless communication with a network (e.g., the Internet, anintranet, a local area network, a mobile communications network, or thelike). In some embodiments, network interface 406 can include anycombination of any number of a network interface controller (NIC), aradio frequency (RF) module, a transponder, a transceiver, a modem, arouter, a gateway, a wired network adapter, a wireless network adapter,a Bluetooth adapter, an infrared adapter, an near-field communication(“NFC”) adapter, a cellular network chip, or the like.

In some embodiments, optionally, apparatus 400 can further includeperipheral interface 408 to provide a connection to one or moreperipheral devices. As shown in FIG. 4 , the peripheral device caninclude, but is not limited to, a cursor control device (e.g., a mouse,a touchpad, or a touchscreen), a keyboard, a display (e.g., acathode-ray tube display, a liquid crystal display, or a light-emittingdiode display), a video input device (e.g., a camera or an inputinterface coupled to a video archive), or the like.

It should be noted that video codecs (e.g., a codec performing process200A, 200B, 300A, or 300B) can be implemented as any combination of anysoftware or hardware modules in apparatus 400. For example, some or allstages of process 200A, 200B, 300A, or 300B can be implemented as one ormore software modules of apparatus 400, such as program instructionsthat can be loaded into memory 404. For another example, some or allstages of process 200A, 200B, 300A, or 300B can be implemented as one ormore hardware modules of apparatus 400, such as a specialized dataprocessing circuit (e.g., an FPGA, an ASIC, an NPU, or the like).

FIG. 5 illustrates a schematic diagram of an exemplary luma mapping withchroma scaling (LMCS) process 500, according to some embodiments of thedisclosure. For example, process 500 can be used by a decoder conformingto a hybrid video coding standard (e.g., H.26x series). The LMCS is anew processing block applied before loop filter 232 of FIG. 2B. The LMCScan also be referred to as a reshaper.

LMCS process 500 can include an in-looping mapping of the luma componentvalues based on an adaptive piecewise linear model and a luma-dependentchroma residual scaling of the chroma components.

As shown in FIG. 5 , the in-looping mapping of the luma component valuesbased on an adaptive piecewise linear model can include a forwardmapping stage 518 and an inverse mapping stage 508. The luma-dependentchroma residual scaling of the chroma components can include chromascaling 520.

Sample values before mapping or after inverse mapping can be referred toas samples in the original domain, and sample values after mapping andbefore inverse mapping can be referred to as samples in the mappeddomain. Some stages in process 500 can be performed in the mapped domaininstead of the original domain, when LMCS is enabled. It is appreciatedthat forward mapping stage 518 and inverse mapping stage 508 can beenabled/disabled at the sequence level using an SPS flag.

As shown in FIG. 5 , Q⁻¹&T⁻¹ stage 504, reconstruction 506, and intraprediction 514 are performed in the mapped domain. For example, Q⁻¹&T⁻¹stage 504 can include inverse quantization and inverse transform,reconstruction 506 can include addition of the luma prediction and theluma residual, and intra prediction 508 can include luma intraprediction.

Loop filters 510, motion compensation stages 516 and 530, intraprediction stage 528, reconstruction stage 522, and decoded picturebuffer (DPB) 512 and 526 are performed in the original (i.e.,non-mapped) domain. In some embodiments, loop filters 510 can includedeblocking, an adaptive loop filter (ALF), and sample adaptive offset(SAO), reconstruction stage 522 can include addition of the chromaprediction together with the chroma residual, and DPB 512 and 526 canstore decoded pictures as reference pictures.

In some embodiments, a method for processing video content using lumamapping with a piecewise linear model can be applied.

The in-loop mapping of the luma component can adjust the signalstatistics of the input video by redistributing the codewords across thedynamic range to improve compression efficiency. Luma mapping makes useof a forward mapping function, “FwdMap” and a corresponding inversemapping function “InvMap.” The “FwdMap” function is signaled using apiecewise linear model with 16 equal pieces. “InvMap” function does notneed to be signaled and is instead derived from the “FwdMap” function.

Signaling of the piecewise linear model is shown in Table 1 of FIG. 6and Table 2 of FIG. 7 ., and later in VVC draft 5, signaling ofpiecewise linear model is changed as in Table 3 of FIG. 8 and Table 4 ofFIG. 9 . Table 1 and Table 3 show syntax structures of the tile groupheader and the slice header. First a reshaper model parameter presenceflag can be signaled to indicate if the luma mapping model is present ina target tile group or a target slice. If the luma mapping model ispresent in the current tile group/slice, piecewise linear modelparameters corresponding to the target tile group or the target slicecan be signaled in tile_group_reshaper_model()/lmcs_data(), using thesyntax elements shown in Table 2 of FIG. 7 and Table 4 of FIG. 9 . Thepiecewise linear model partitions the input signal’s dynamic range into16 equal pieces. For each piece, its linear mapping parameters can beexpressed using a number of codewords assigned to the piece. In anexample of a 10-bit input, each of the 16 pieces of the input can have64 codewords assigned to the piece by default. A number of signaledcodewords can be used to calculate a scaling factor and adjust themapping function accordingly for the piece. Table 2 of FIG. 7 and Table4 of FIG. 9 also define a minimum index and a maximum index among thenumber of signaled codewords, such as “reshaper-model_min_bin_idx” and“reshaper_model_delta­_max_bin_idx” as in Table 2, “lmcs_min_bin_idx” and“lmcs_delta_max_bin_idx” as in Table 4, inclusively. If the piece indexis smaller than “reshaper_model_min_bin_idx” or “lmcs_min_bin_idx” orlarger than “15-reshaper_model_max_bin_idx” or“15-lmcs_delta_max_bin_idx,” then the number of codewords for that pieceis not signaled and is inferred to be zero. In other words, no codewordsare assigned and no mapping/scaling is applied to the piece.

At the tile group header level or slice header level, another reshaperenable flag (e.g., “tile_group_reshaper_enable_flag” or“slice_lmcs_enabled_flag”) can be signaled to indicate if the LMCSprocess as depicted in FIG. 5 is applied to the target tile group or thetarget slice. If a reshaper is enabled for the target tile group or thetarget slice and if the target tile group or the target slice does notuse dual tree partition, then a further chroma scaling enable flag canbe signaled to indicate if chroma scaling is enable for the target tilegroup or the target slice. It is appreciated that dual tree partition iscan also be referred to as chroma separate tree. In the following, thepresent disclosure will explain dual tree partition in more detail.

The piecewise linear model can be constructed based on the signaledsyntax elements in Table 2 or Table 4 as follows. An i-th piece (i = 0... 15) of the “FwdMap” piecewise linear model can be defined by twoinput pivot points InputPivot[] and two mapped pivot pointsMappedPivot[]. The mapped pivot points MappedPivot[] can be an output of“FwdMap” piecewise linear model. The InputPivot[] and MappedPivot[] canbe computed based on the signaled syntax as follows, assuming the bitdepth of the exemplary input video is 10-bit. It is appreciated that thebit depth can be different from 10-bit.

-   a) Using syntax elements in Table 2:    -   1) OrgCW = 64    -   2) For i = 0:16, InputPivot[ i ] = i * OrgCW    -   3) For i = reshaper-model_min_bin_idx :        reshaper_model_max_bin_idx, SignaledCW[ i ] = OrgCW + (1 ¬        2*reshape_model_bin_delta_sign_CW [ i ]) *        reshape_model_bin_delta_abs_CW [ i ];    -   4) For i = 0: 16, MappedPivot[i] is calculated as follows:        -   MappedPivot[ 0 ] = 0;        -   for(i = 0; i <16; i++) MappedPivot[ i + 1 ] = MappedPivot[ i            ] + SignaledCW[ i ]-   b) Using syntax elements in Table 4:    -   1) OrgCW = 64    -   2) For i = 0:16, InputPivot[ i ] = i * OrgCW    -   3) For i = lmes_min_bin_idx : lmesl_max-bin_idx, SignaledCW[ i ]        = OrgCW + (1 ¬ 2*lmcs_bin_delta_sign_CW [ i ]) *        lmcsl_bin_delta_abs_CW [ i ];    -   4) For i = 0: 16, MappedPivot[i] is calculated as follows:        -   MappedPivot[ 0 ] = 0;        -   for(i = 0; i <16; i++) MappedPivot[ i + 1 ] = MappedPivot[ i            ] + SignaledCW[ i ]

The inverse mapping function “InvMap” is also defined by InputPivot[]and MappedPivot[]. Different from “FwdMap,” for the “InvMap” piecewiselinear model, the two input pivot points of each piece are defined byMappedPivot[] and the two output pivot points are defined byInputPivot[]. In this way, the input of the “FwdMap” is partitioned intoequal pieces, but the input of the “InvMap” is not guaranteed to bepartitioned into equal pieces.

As shown in FIG. 5 , for an inter-coded block, motion compensatedprediction can be performed in the mapped domain. In other words, afterthe motion-compensated prediction 516, Y_(pred) is calculated based onthe reference signals in the DPB, the “FwdMap” function 518 can beapplied to map the luma prediction block in the original domain to themapped domain, Y′_(pred) = FwdMap(Y_(pred)). For an intra-coded block,the “FwdMap” function is not applied because the reference samples usedin intra prediction are already in the mapped domain. Afterreconstructed block 506, Y_(r) can be calculated. The “InvMap” function508 can be applied to convert the reconstructed luma values in themapped domain back to the reconstructed luma values in the originaldomain (Y _(i) = InvMap(Y_(r))). The “InvMap” function 508 can beapplied to both intra- and inter-coded luma blocks.

The luma mapping process (forward or inverse mapping) can be implementedusing either look-up-tables (LUT) or using on-the-fly computation. IfLUT is used, then tables “FwdMapLUT[]” and “InvMapLUT[]” can bepre-calculated and pre-stored for use at the tile group level or theslice level, and forward and inverse mapping can be simply implementedas FwdMap(Y_(pred)) = FwdMapLUT[Y_(pred)] and InvMap(Y_(r)) =InvMapLUT[Y_(r)], respectively.

Alternatively, on-the-fly computation can be used. Take forward mappingfunction “FwdMap” as an example. In order to determine a piece to whicha luma sample belongs, the sample value can be right shifted by 6 bits(which corresponds to 16 equal pieces assuming 10-bit video) to obtainthe piece index. Then, the linear model parameters for that piece areretrieved and applied on-the-fly to compute the mapped luma value. The“FwdMap” function is evaluated as follows:

Y^(′)_(pred) = FwdMap(Y_(pred)) = ((b2 − b1)/(a2 − a1)) * (Y_(pred) − a1) + b1

wherein “i” is the piece index, a1 is InputPivot[i], a2 isInputPivot[i+1], b1 is MappedPivot[i], b2 is MappedPivot[i+1].

The “InvMap” function can be computed on-the-fly in a similar manner,except that conditional checks need to be applied instead of a simpleright bit-shift when figuring out the piece to which the sample valuebelongs, because the pieces in the mapped domain are not guaranteed tobe equal sized.

In some embodiments, a method for process video content usingluma-dependent chroma residual scaling can be provided.

Chroma residual scaling can be used to compensate for the interactionbetween a luma signal and chroma signals corresponding to the lumasignal. Whether chroma residual scaling is enabled can also be signaledat the tile group level or the slice level. As shown in Table 1 of FIG.6 and Table 3 of FIG. 8 , if luma mapping is enabled and if dual treepartition is not applied to the current tile group, an additional flag(e.g., “tile_group_reshaper_chroma_residual_scale_flag” or“slice_chroma_residual_scale_flag”) can be signaled to indicate ifluma-dependent chroma residual scaling is enabled. When luma mapping isnot used or dual tree partition is used in the target tile group(or thetarget slice), luma-dependent chroma residual scaling can be disabledaccordingly. Further, luma-dependent chroma residual scaling can bedisabled for chroma blocks whose area is less than or equal to 4.

Chroma residual scaling depends on an average value of a luma predictionblock (for both intra- and inter-coded blocks) corresponding to thechroma signals. The average of the luma prediction block “avgY’” can bedetermined using an equation below.

$avgY^{\prime} = \frac{\sum_{i = 0}^{width - 1}\sum_{j = 0}^{height - 1}\text{predSamples}\left\lbrack \text{i} \right\rbrack\left\lbrack \text{j} \right\rbrack}{width \ast height}$

A value of a chroma scaling factor for chroma residual scaling“C_(scaleInv)” can be determined using steps as below.

-   1) Find the index Y_(Idx) of the piecewise linear model to which    avgY’ belongs based on the InvMap function.-   2) C_(ScaleInv) = cScaleInv[Y_(Idx)], where cScaleInv[] is a    pre-computed LUT having e.g., 16 pieces.

In some embodiments, in the LMCS method, the pre-computed LUT“cScaleInv[i]” with i in a range of 0 to 15 can be derived based on a64-entry static LUT “ChromaResidualScaleLut” and signaled codewords“SignaledCW[i]” value, as below.

-   ChromaResidualScaleLut[64] = {16384, 16384, 16384, 16384, 16384,    16384, 16384, 8192, 8192, 8192, 8192, 5461, 5461, 5461, 5461, 4096,    4096, 4096, 4096, 3277, 3277, 3277, 3277, 2731, 2731, 2731, 2731,    2341, 2341, 2341, 2048, 2048, 2048, 1820, 1820, 1820, 1638, 1638,    1638, 1638, 1489, 1489, 1489, 1489, 1365, 1365, 1365, 1365, 1260,    1260, 1260, 1260, 1170, 1170, 1170, 1170, 1092, 1092, 1092, 1092,    1024, 1024, 1024, 1024};-   shiftC = 11    -   if ( SignaledCW [ i ] == 0 ) cScaleInv [ i ] = (1 << shiftC)    -   Otherwise, cScaleInv [ i ] = ChromaResidualScaleLut[ (SignaledCW        [ i ] >> 1)-1]

As an example, assuming the input is 10-bit, the static LUT“ChromaResidualScaleLut[]” contains 64 entries, and the signaledcodewords “SignaledCW[]” is in a range of [0, 128]. Therefore, a divideby 2 (or right shift by 1) is used to construct the chroma scalingfactor LUT “cScaleInv []”. The LUT “cScaleInv []” can be constructed atthe tile group (or the slice level).

If the current block can be coded using intra, CIIP, or intra block copy(IBC) mode, avgY’ can be determined as the average of the intra-, CIIP-,or IBC- predicted luma values. Otherwise, avgY’ is computed as theaverage of the forward mapped inter predicted luma values (that is,Y′_(pred) in FIGS. 3 )., The IBC can also be referred to as a currentpicture referencing (CPR) mode. Unlike luma mapping, which is performedon the sample basis, “C_(ScaleInv)” is a constant value for the entirechroma block. With “C_(ScaleInv)”, chroma residual scaling can beapplied at the decoder side as follows: Decoder side: C _(Res) = C_(ResScale) * C_(ScaleInv)

Where C _(ResScale) is the reconstructed chroma residual of the currentblock. At the encoder side, the forward chroma residual scaling (beforebeing transformed and quantized) is performed as follows: Encoder side:C _(ResScale) = C_(Res) * C_(Scale) = C_(Res)/C_(ScaleInv).

In some embodiments, a method for processing video content usingcross-component linear model prediction can be provided.

To reduce the cross-component redundancy, a cross-component linear model(CCLM) prediction mode can be used. In CCLM, chroma samples arepredicted based on the reconstructed luma samples of a same coding unit(CU) by using a linear model as follows:

pred_(C)(i, j) = α ⋅ rec_(L)^(′)(i, j) + β

where pred_(C)(i, j) represents the predicted chroma samples in a CU andrec_(L)(i, j) represents the downsampled reconstructed luma samples ofthe same CU.

Linear model parameter α and β are derived based on the relation betweenluma values and chroma values from two sample positions. The two samplepositions can include, among a set of downsampled neighboring lumasamples, a first luma sample position with a maximum luma sample valueand a second luma sample position with a minimum luma sample value, andtheir corresponding chroma samples. The linear model parameters α and βare obtained according to the following equations.

$\text{a} = \frac{Y_{a} - Y_{b}}{\text{X}_{a} - X_{b}}$

β = Y_(b) − a ⋅ X_(b)

Where Y_(a) and X_(a) represent luma value and chroma value of firstluma sample position, respectively. And X_(b) and Y_(b) represent lumavalue and chroma value of the second luma sample position, respectively.

FIG. 10 illustrates an example of sample positions involved in the CCLMmode, according to some embodiments of the disclosure.

The calculation of parameter α can be implemented with a look-up table.To reduce the memory required for storing the table, the diff value(difference between maximum and minimum values) and the parameter α areexpressed by an exponential notation. For example, diff is approximatedwith a 4-bit significant part and an exponent. Consequently, the tablefor 1/diff is reduced into 16 elements for 16 values of the significandas follows:

DivTable [] = {0, 7, 6, 5, 5, 4, 4, 3, 3, 2, 2, 1, 1, 1, 1, 0}

The table “DivTable []” can reduce the complexity of the calculation andalso reduce the memory size required for storing the needed tables

Besides the top positions and left positions can be used to calculatethe linear model coefficients together, they also can be usedalternatively in the other 2 LM modes, called LM_A, and LM_L modes.

In LM_A mode, only samples at top positions are used to calculate thelinear model coefficients. To acquire more samples, the top positionscan be extended to cover (W+H) samples. In LM_L mode, only samples atleft positions are used to calculate the linear model coefficients. Toget more samples, the left positions can be extended to cover (H+W)samples.

For a non-square block, the above template are extended to W+W, the lefttemplate are extended to H+H.

To match the chroma sample locations for 4:2:0 video sequences, twotypes of downsampling filters can be applied to luma samples to achieve2 to 1 downsampling ratio in both horizontal and vertical directions.The selection of a downsampling filter can be specified by a SPS levelflag. The two downsampling filters are as follows, which arecorresponding to “type-0” and “type-2” content, respectively.

$\text{rec}_{\text{L}}{}^{\prime}\left( {i,j} \right) = \begin{bmatrix}{\text{rec}_{\text{L}}\left( {2i - 1,2j - 1} \right) + 2 \cdot \text{rec}_{\text{L}}\left( {2i - 1,2j - 1} \right) + \text{rec}_{\text{L}}\left( {2i + 1,2j - 1} \right) +} \\{\text{rec}_{\text{L}}\left( {2i - 1,2j} \right) + 2 \cdot \text{rec}_{\text{L}}\left( {2i,2j} \right) + \text{rec}_{\text{L}}\left( {2i + 1,2j} \right) + 4}\end{bmatrix} \gg 3$

$\text{rec}_{\text{L}}{}^{\prime}\left( {i,j} \right) = \begin{bmatrix}{\text{rec}_{\text{L}}\left( {2i,2j - 1} \right) + \text{rec}_{\text{L}}\left( {2i - 1,2j} \right) + 4 \cdot \text{rec}_{\text{L}}\left( {2i,2j} \right)} \\{+ \text{rec}_{\text{L}}\left( {2i + 1,2j} \right) + \text{rec}_{\text{L}}\left( {2i,2j + 1} \right) + 4}\end{bmatrix} \gg 3$

It is appreciated that only one luma line (general line buffer in intraprediction) is used to compute the downsampled luma samples when theupper reference line is at the CTU boundary.

This parameter computation can be performed as part of the decodingprocess, and is not just as an encoder search operation. As a result, nosyntax is used to convey the α and β values to the decoder. The α and βparameters are computed for each of the chroma components, separately.

For chroma intra mode coding, a total of 8 intra modes can be allowed.Those modes include five traditional intra modes and threecross-component linear model modes (e.g., CCLM, LM_A, and LM_L). Aprocess for signalling and deriving a chroma mode when CCLM is enabledis shown in Table 5 of FIG. 9 . Chroma mode coding of a chroma block candepend on the intra prediction mode of a luma block corresponding to thechroma block. Because separate block partitioning structures for lumaand chroma components are enabled in I slices (which will be describedbelow), one chroma block can correspond to multiple luma blocks.Therefore, for chroma derived mode (DM), the intra prediction mode ofthe corresponding luma block covering the center position of the currentchroma block is inherited.

In some embodiments, a method for processing video content using a dualtree partition can be provided.

In VVC draft, the coding tree scheme supports the ability for the lumaand chroma to have separate block tree partitions. This is also referredto as dual tree partition. In VVC draft, signaling of dual treepartition is shown in Table 6 of FIG. 12 ) and Table 7 of FIG. 13 . Andlater in VVC draft 5, dual tree partition is signaled as in Table 8 ofFIG. 14 and Table 9 of FIG. 15 ). When a sequence level control flagsignaled in the SPS (e.g., “qtbtt_dual_tree_intra_flag”) is turned onand when a target tile group (or a target slice) is intra coded, blockpartition information can be signaled first for luma and then forchroma, separately. For inter coded tile groups/slices (e.g., P and Btile groups/slices), dual tree partition is not allowed. When separateblock tree modes are applied, luma coding tree block (CTB) ispartitioned into CUs by a first coding tree structure, and the chromaCTBs are partitioned into chroma CUs by a second coding tree structure,as shown in Table 7 of FIG. 13 .

When luma and chroma blocks are allowed to have different partitions,problems may arise for coding tools that have dependency between thedifferent color components. For example, when LMCS is applied, anaverage value of a luma block corresponding to a target chroma block canbe used to determine a scaling factor to be applied on the target chromablock. When dual tree partition is used, determination of the averagevalue of the luma block can create a latency of an entire CTU. Forexample, if the luma block of the CTU is split vertically once, and thechroma block of the CTU is split horizontally once, then both of theluma blocks of the CTU are decoded for calculating the average value,before the first chroma block of the CTU can be decoded. In VVC, the CTUcan be as large as 128×128 in units of luma samples, causing latency ofdecoding the chroma block to increase significantly. Therefore, VVCdraft 4 and draft 5 can prohibit the combination of dual tree partitionand luma-dependent chroma scaling. When dual tree partition is enabledfor the target tile group (or target slice), chroma scaling can beforced to be off. Note that the luma mapping part of LMCS is stillallowed in dual tree partition, as it operates only on the lumacomponents and has no cross color component dependency problem.

Another example of a coding tool that relies on the dependency betweenthe color components to achieve better coding efficiency is called thecross component linear model (CCLM), which has been discussed above. InCCLM, neighboring luma and chroma reconstructed samples can be used toderive cross component parameters. And the cross component parameterscan be applied to the corresponding reconstructed luma sample of thetarget chroma block to derive predictors for the chroma components. Whendual tree partition is used, the luma and chroma partitions are notguaranteed to be aligned. Thus, CCLM cannot be started for a chromablock until all of the corresponding luma blocks that contain samplesused in CCLM have been reconstructed.

FIGS. 16A-16B illustrate an exemplary chroma tree partition and anexemplary luma tree partition, according to some embodiments of thedisclosure. FIG. 16A illustrates an exemplary partitioning structure ofa chroma block 1600. And FIG. 16B illustrates an exemplary partitioningstructure of a luma block 1610 corresponding to chroma block 1600 ofFIGS. 16 . In FIG. 16A, chroma block 1600 is quad-split into 4sub-blocks and the bottom-left sub-block is further quad-split to 4sub-blocks and the block with a grid pattern is current block to bepredicted. In FIG. 16B, luma block 1610 is horizontally bi-split to 2sub-blocks, the area with a grid pattern is the area corresponding tothe target chroma block to be predicted. To derive the CCLM parameters,the neighboring reconstructed sample values represented by emptycirculars are needed. Thus, the prediction of target chroma block cannotbe started until the bottom luma block reconstruction is finished whichintroduces a large latency.

In some embodiments, a method for processing video content using virtualpipeline data units can be provided.

In VVC standardization, a concept of virtual pipeline data units (VPDUs)is introduced for more friendly hardware implementation. VPDUs aredefined as non-overlapping M×M-luma(L)/N×N-chroma(C) units in a picture.In hardware decoders, successive VPDUs are processed by multiplepipeline stages at the same time. Different stages process differentVPDUs simultaneously. The VPDU size is roughly proportional to thebuffer size in most pipeline stages, and therefore, it is important tokeep the VPDU size small. In VVC, the size of VPDU is set to 64×64samples. Thus, all the coding tools adopted in VVC cannot violate theVDPU restriction. For example, the maximum transform size can only be64×64, as the whole transform block need to be operated at same pipelinestage. Because of the VPDU restriction, the intra prediction blockshould also be no larger than 64×64. Therefore, in intra coded tilegroups/slices (e.g., I tile groups/slices), the CTU is forced to besplit into 4 64×64 blocks (if CTU is larger than 64×64) and each 64×64block can be further split with a dual tree structure. Thus, a commonroot of the luma partitioning tree and the chroma partitioning tree isat the 64×64 block size when dual tree is enabled.

There are several problems in the current design of LMCS and CCLM.

First, for example, derivation of a tile group level chroma scalingfactor LUT “cScaleInv[]” is not easily extensible. The derivationprocess currently depends on a constant chroma LUT“ChromaResidualScaleLut” with 64 entries. For a 10-bit video with 16pieces, an additional step of division by 2 must be applied. When thenumber of pieces changes (for example, if 8 pieces instead of 16 piecesare used), then the derivation process must be changed to apply adivision by 4 instead of 2. This additional step can cause precisionloss.

Second, for example, to calculate the Y_(Idx), which is used to obtainthe chroma scaling factor, the average value of the entire luma block isused. Considering the maximum CTU size of 128×128, the average lumavalue can be calculated based on 16384 (128×128) luma samples, which iscomplex. Further, if a luma block partition of 128×128 is selected bythe encoder, that block is more likely to contain homogenous content.Therefore, a subset of the luma samples in the block can be sufficientfor calculating the luma average.

Third, during dual tree partition, the chroma scaling is set to be offto avoid potential pipeline issues for hardware decoders. However, thisdependency can be avoided if explicit signaling is used to indicate thechroma scaling factor to be applied (instead of using the correspondingluma samples to derive it). Enabling chroma scaling in intra coded tilegroups/slices can further improve coding efficiency.

Fourth, conventionally, a delta codeword value is signaled for each ofthe 16 pieces. It has been observed that often only a limited number ofdifferent codewords is used for the 16 pieces. Therefore, signalingoverhead may be further reduced.

Fifth, the parameters of CCLM are derived with luma and chromareconstructed samples from the blocks that are causal neighbors oftarget chroma blocks. In dual tree partitions, the luma and chroma blockpartitions are not necessarily aligned. Therefore, more than one lumablock or a luma block with larger area than a target chroma block cancorrespond to the target chroma block. To derive the CCLM parameters ofthe target chroma block, all of the corresponding luma blocks must bereconstructed first as shown in FIGS. 16A-16B. This incurs latency inpipeline implementation and decreases the throughput of a hardwaredecoder.

To Address the above problems, embodiments of the disclosure areprovided as below.

Embodiments of the disclosure provide a method for processing videocontent by removing chroma scaling LUT.

As mentioned above, the chroma LUT of 64 entries is not easilyextensible and can cause problems when other piecewise linear models areused (e.g., 8 pieces, 4 pieces, 64 pieces, etc). It is also unnecessarybecause the chroma scaling factor can be set the same as the lumascaling factor of that corresponding piece to achieve the same codingefficiency. In some embodiments of this disclosure, denote Y_(Idx) asthe piece index of the current chroma block, the following steps areused to determine the chroma scaling factor:

-   if Y_(Idx) > reshaper_model_max_bin_idx or Y_(Idx) <    reshaper_model_min_bin_idx, or if SignaledCW[Y_(Idx)] = 0, then set    chroma_scaling to default, chroma_scaling = 1.0, i.e., no scaling is    applied.-   Otherwise, set chroma_scaling to SignaledCW[Y_(Idx)]/OrgCW.

The chroma scaling factor derived above has fractional precision. Fixedpoint approximation can be applied to avoid dependency onhardware/software platforms. Also, at the decoder side, inverse chromascaling needs to be performed. Such division can be implemented by fixedpoint arithmetic using multiplication followed by right shift. Denotethe number of bits in the fixed point approximation as CSCALE_FP_PREC.The following may be used to determine the inverse chroma scaling factorin fixed point precision:

-   inverse_chroma_scaling[Y_(Idx)] = ((1 << (luma_bit_depth    -log2(TOTAL_NUMBER_PIECES) + CSCALE_FP_PREC)) + (SignaledCW[Y_(Idx)]    >>    -   1)) / SignaledCW [Y_(Idx)]; where luma_bit_depth is the luma bit        depth, TOTAL_NUMBER_PIECES is total number of pieces in the        piecewise linear model, which is set to 16 in VVC draft 4. Note        that inverse_chroma_scaling values may only need to be        calculated once per tile group/slice, and the division in the        above is an integer division operation.

Further quantization may be applied to derive the chroma scaling andinverse scaling factor. For example, the inverse chroma scaling factormay be calculated for all the even (2×m) values of SignaledCW, and theodd (2×m+1) values of the SignaledCW reuses the chroma scaling factor ofthe neighboring even value’s scaling factor. In other words, thefollowing may be used:

-   for(i = reshaper_model_min_bin_idx; i <= reshaper_model_max_bin_idx;    i++) { tempCW = SignaledCW[i] >> 1)<<1;-   inverse_chroma_scaling[i] = ((1 << (luma_bit_depth -    log2(TOTAL_NUMBER_PIECES) + CSCALE_FP_PREC)) + (tempCW >> 1)) /    tempCW; }

The above embodiments of quantizing the chroma scaling factors can befurther generalized, for example, the inverse chroma scaling factor maybe calculated for every n-th value of SignaledCW, with all otherneighboring values sharing the same chroma scaling factor. For example,“n” may be set to 4, meaning that every 4 neighboring codeword valuesshare the same inverse chroma scaling factor value. It is desirable forthe value of “n” to be a power of 2, which allows shifting to be used tocalculate division. Denote the value of log2(n) as LOG2_n, the above canbe modified as follows: tempCW = SignaledCW[i] >> LOG2_n)<<LOG2_n.

Finally, the value of LOG2_n may be a function of the number of piecesused in the piecewise linear model. It is beneficial to use a largerLOG2_n if fewer pieces are used. For example, LOG2_n can be set to1+(4 - log2(TOTAL_NUMBER_PIECES)) if the value of TOTAL_NUMBER_PIECES issmaller or equal to 16. If TOTAL_NUMBER_PIECES is larger than 16, thenLOG2_n can be set to 0.

Embodiments of the disclosure provide a method for processing videocontent by simplifying the averaging of luma prediction block.

As discussed above, to determine a piece index of the current chromablock “Y_(Idx),” the average value of the corresponding luma block canused. However, for large block sizes, the averaging process can involvea large number of luma samples. In a worst case, 128×128 luma samplescan be involved in the averaging process.

Embodiments of the disclosure provide a simplified averaging process toreduce the worst case to only using N×N luma samples (N is a power of2).

In some embodiments, if not both dimensions of a two-dimensional lumablock are less than or equal to a preset threshold N (in other words, atleast one of the two dimension is larger than N,) a “downsampling” canbe applied to use only N positions in that dimension. Without loss ofgenerality, take the horizontal dimension as an example. If width islarger than N, then only samples at position x, x = i×(width >>log2(N)), i = 0,... N-1, are used in averaging.

FIG. 17 illustrates an exemplary simplification of an averagingoperation, according to some embodiments of the disclosure. In thisexample, N is set to 4, and only 16 luma samples (the shaded samples) inthe block are used in averaging. It is appreciated that the value of Nis not limited to 4. For example, N can be set to be any values that arepowers of 2. In other words, N may be 1, 2, 4, 8, etc.

In some embodiments, different values of N may be applied in thehorizontal and the vertical dimension. In other words, the worst case ofthe averaging operation can be using N×M samples. In some embodiments,the number of samples can be limited in the averaging process withoutconsidering the dimension. For example, a maximum of 16 samples can beused. The 16 samples can be distributed in the horizontal or verticaldimension in a form of 1×16, 16×1, 2×8, 8×2, 4×4, or a form that fitsthe shape of the target block. For example, 2×8 is used if the block isnarrow and tall, 8×2 is used if the block is wide and short, and 4×4 isused when the block is square.

Although such a simplification can cause the average value to bedifferent from the true average of the entire luma block, any suchdifference is likely small. This is because when large block size isselected, the content within the block tends to be more homogeneous.

Moreover, the decoder-side motion vector refinement (DMVR) mode is acomplicated process in the VVC standard, especially for the decoder.This is because DMVR requires the decoder to perform motion search toderive the motion vector, before motion compensation can be applied. Thebi-directional optical flow (BDOF) mode in the VVC standard can furthercomplicate the situation, because BDOF is an additional sequentialprocess that needs to be applied after DMVR, in order to obtain a lumaprediction block. Because chroma scaling requires the average value ofthe corresponding luma prediction block, DMVR and BDOF can be appliedbefore the average value can be calculated, causing a latency issue.

To solve this latency issue, in some embodiments of this disclosure, theluma prediction block is used before DMVR and BDOF to calculate theaverage luma value, and the average luma value is used to obtain thechroma scaling factor. This allows chroma scaling to be applied inparallel to the DMVR and BDOF processes, and therefore can significantlyreduce latency.

Consistent with the present disclosure, variations of the latencyreduction can be contemplated. In some embodiments, this latencyreduction can also be combined with the simplified averaging processdiscussed above that uses only a portion of the luma prediction block tocalculate the average luma value. In some embodiments, the lumaprediction block can be used after the DMVR process and before the BDOFprocess to calculate the average luma value. The average luma value isthen used to obtain the chroma scaling factor. This design allows chromascaling to be applied in parallel to the BDOF process while keeping theaccuracy of determining the chroma scaling factor. The DMVR process canrefine the motion vector, and therefore, using the prediction samplewith the refined motion vector after the DMVR process can be moreaccurate than using the prediction sample with the motion vector beforethe DMVR process.

Moreover, in the VVC standard, the CU syntax structure (e.g.,coding_unit()) can include a syntax element “cu_cbf” to indicate ifthere is any non-zero residual coefficients in a target CU. At the TUlevel, the TU syntax structure transform_unit() includes syntax elementstu_cbf_cb and tu_cbf_cr to indicate if there is any non-zero chroma (Cbor Cr) residual coefficients in the target TU. In the VVC draft 4, theaveraging of the corresponding luma block can be invoked if chromascaling is enabled at the tile group level or the slice level. Thepresent disclosure also provides a method to bypass the luma averagingprocess. Consistent with the disclosed embodiments, because the chromascaling process is applied to the residual chroma coefficients, the lumaaveraging process can be bypassed if there is no non-zero chromacoefficients. This can be determined based on the following conditions:

-   Condition 1: cu_cbf is equal to 0-   Condition 2: tu_cbf_cr and tu_cbf_cb are both equal to 0

When either Condition 1 or Condition 2 is met, the luma averagingprocess can be bypassed.

In the above embodiment, only N×N samples of the prediction block areused to derive the average value, which simplifies the averagingprocess. For example, when N is equal to 1, only the top left sample ofthe prediction block can be used. However, even this simplified caserequires the prediction block to be generated first, thereby creatinglatency. Therefore, in some embodiments, it contemplated that thereference luma samples can be used directly to derive the chroma scalingfactor. This allows the decoder to derive the chroma scaling factor inparallel to the luma prediction process, thus reducing latency. In otherwords, the intra prediction and the inter prediction are processedseparately.

In the case of intra prediction, the already decoded neighboring samplesin a same picture can be used as reference samples to generate theprediction block. These reference samples include samples on the top ofthe target block, to the left of the target block, and to the top-leftof the target block. The average of all these reference samples can beused to derive the chroma scaling factor. Alternatively, the average ofonly a portion of these reference samples can be used. For example, onlyM reference samples (e.g., M=3) closest to the top-left position of thetarget block can be averaged.

As another example, the M reference samples which are averaged to derivethe chroma scaling factor are not closest to the top-left position butare distributed along the top boundary and left boundary of the targetblock as in FIG. 18 . FIG. 18 illustrates exemplary samples used in anaverage calculation to derive the chroma scaling factor. As shown inFIG. 18 , the exemplary samples are represented in solid dashed boxes.One, two, three, four, five, six and eight samples are averaged in eachof exemplary blocks 1801-1807 shown in FIG. 18 . The average calculationin this disclosure can be replaced with a weighted average wheredifferent samples can have different weights in the average calculation.For example, a sum of weights can be a power of 2 to avoid divisionoperation in the average calculation.

In the case of inter prediction, reference samples from temporalreference pictures can be used to generate the prediction block. Thesereference samples are identified by the reference picture indices andthe motion vectors. Interpolation can be applied if the motion vectorshave fractional precision. To calculate the average of referencesamples, the reference samples after interpolation may be used, or thereference samples before interpolation (that is, motion vectors that areclipped to integer precision) may also be used. Consistent with thedisclosed embodiments, all of the reference samples may be used tocalculate the average. Alternatively, only a portion of the referencesamples (e.g., reference samples corresponding to the top-left positionof the target block) may be used to calculate the average.

As shown in FIG. 5 , intra prediction is performed in the reshapeddomain, while inter prediction is performed in the original domain.Therefore, for inter prediction, a forward mapping is applied on theprediction block, and the luma prediction block after forward mapping isused to calculate an average value of the luma block. To reduce latency,the average value can be calculated using the luma prediction blockbefore forward mapping. For example, the whole luma block before forwardmapping, or a N×N portion of the luma block before forward mapping, orthe top-left sample of the luma block before forward mapping can beused.

Embodiments of the disclosure further provide a method for processingvideo content with chroma scaling for dual-tree partitions.

Because the dependency on the luma block can cause hardware designcomplications, chroma scaling can be turned off for intra-coded tilegroups/slices that enable dual-tree partitions. However, thisrestriction can cause coding efficiency loss.

Because a CTU is a common root of both a luma coding tree and a chromacoding tree, deriving a chroma scaling factor on CTU level can removethe dependency between chroma and luma in dual tree partitions. Forexample, the CTU neighboring reconstructed luma samples or chromasamples are used to derive a chroma scaling factor. Then, this chromascaling factor can be used for all the chroma samples within the CTU. Inthis example, the method of averaging reference samples above can beapplied to average the CTU neighboring reconstructed samples. Theaverage of all these reference samples can be used to derive the chromascaling factor. Or, the average value of only a portion of thesereference samples can be used. For example, only M reference samples(e.g., M=4, 8, 16, 32 or 64) closest to the top-left position of thetarget block can be averaged.

However, for the CTUs on the picture bottom or right boundary, all thesamples of the CTUs may not be within the picture boundary as grey CTUsin FIG. 19 . In this case, only the neighboring reconstructed samples onthe CTU boundary within the picture boundary (grey samples in FIG. 19 )can be used to derive the chroma scaling factor. But a variable numberof samples in average calculation requires a division operation which isundesirable in hardware implementation. Therefore, embodiments of thedisclosure provide a method of padding the picture boundary samples to afixed number that is a power of 2, so that the division operation inaverage calculation can be avoided. For example, as shown in FIG. 19 ,the padded samples out of a picture bottom boundary (white samples inFIG. 19 ) are generated from a sample 1905 that is a sample that isclosest to the padded samples among all the samples on the picturebottom boundary. In additional to the CTU level chroma scaling factorderivation, the chroma scaling factor can be derived on a fixed grid.Considering the Virtual Pipeline Data Unit (VPDU) which is defined as adata unit processed by pipeline stages, a chroma scaling factor can bederived in VPDU level. In the VVC draft 5, VPDU is defined as 64×64blocks on the luma sample grid. Therefore, chroma scaling factorderivation at the granularity of 64×64 blocks is provided in embodimentsof this disclosure. In the VVC draft 6, VPDU is defined as M×M blocks onthe luma sample gird wherein M is the smaller one of a CTU size and 64.The CTU level derivation method explained before can also be used at theVPDU level.

In some embodiments, on top of deriving the chroma scaling factor on afixed grid with a grid size being smaller than a CTU, the factor isderived only once per CTU and used for all grid units (e.g., VPDU)within the CTU. For example, deriving the chroma scaling factor on thefirst VPDU of a CTU and the factor is used for all the VPDUs within theCTU. It is appreciated that the method on the VPDU level is equivalentto the CTU level derivation using a limited number of neighboringsamples in the derivation (e.g., using only neighboring samplescorresponding to the first VPDU in the CTU).

Instead of averaging the sample values of a corresponding luma block tocalculate avgY’ at the CTU level, at the VPDU level, or at any otherfixed-size block unit level, determining the piece index Y_(ldx), andobtaining the chroma scaling factor inverse_chroma_scaling[Y_(ldx)], thechroma scaling factor can also be explicitly signaled in the bitstreamto avoid the dependency on luma in the case of dual tree partition.

The chroma scaling index can be signaled at a plurality of levels. Forexample, the chroma scaling index can be signaled at the coding unit(CU) level, together with the chroma prediction mode, as shown in Table10 of FIG. 20 and Table 11 of FIG. 21 . The syntax elementlmcs_scaling_factor_idx (element 2002 in FIG. 20 and element 2102 inFIG. 21 ) is used to determine the chroma scaling factor for the targetchroma block. When it is not present, the chroma scaling factor for thetarget chroma block is inferred to be equal to 1.0 in floating pointprecision or equivalently (1 << CSCALE_FP_PREC) in fixed pointprecision. The range of allowed values for lmcs_chroma_scaling_idx canbe determined at a tile group level or a slice level, and will bediscussed later.

Depending on the possible values of lmcs_chroma_scaling_idx, itssignaling cost can be too high, especially for small blocks. Therefore,in some embodiments of this disclosure, the signaling condition in Table10 of FIG. 20 can additionally include block size conditions. Forexample, this syntax element of “lmcs_chroma_scaling_idx” (element 2002in FIG. 20 ) is only signaled, if the target block contains more than Nchroma samples or if the target block has a width larger than a givenwidth W and/or a height larger than a given height H. For smallerblocks, if the lmcs_chroma_scaling_idx is not signaled, then its chromascaling factor can be determined at the decoder side. As an example, thechroma scaling factor can be set to 1.0 in floating point precision. Insome embodiments, a default lmcs_chroma_scaling_idx value can be addedat the tile group header level or the slice header level (Table 1 ofFIG. 10 ). Blocks that having no signaled lmcs_chroma_scaling_idx (e.g.,small blocks) can use the tile group/slice level default index to derivea chroma scaling factor corresponding to the blocks. In someembodiments, the chroma scaling factor of a small block can be inheritedfrom neighbors (e.g., top or left neighbors) of the small block, whichhave explicitly signaled scaling factors.

Besides signaling this syntax element of “lmcs_chroma_scaling_idx” atthe CU level, it can also be signaled at the CTU level. However, whenthe maximum CTU size is 128×128 in VVC, chroma scaling according to thesignaled syntax element of “lmcs_chroma_scaling_idx” at the CTU levelcan be too coarse. Therefore, in some embodiments of this disclosure,this syntax element of “lmcs_chroma_scaling_idx” can be signaled usingfixed granularity. For example, for an area with 16×16 samples (or, anarea with 64×64 samples for VPDU), one lmcs_chroma_scaling_idx cansignaled and applied to samples in the area with 16×16 samples (or, thearea with 64×64 samples).

The range of lmcs_chroma_scaling_idx for the target tile group/slicedepends on how many chroma scaling factor values are allowed in thetarget tile group/slice. The range of lmcs_chroma_scaling_idx can bedetermined by the existing method in VVC that relies on the 64-entrychroma LUT as discussed above. Alternatively, it can also be determinedusing the chroma scaling factor calculation discussed above.

As an example, the value of LOG2_n is set to 2 (i.e., “n” is set to 4)for the “quantization” method described above, and the codewordassignment of each piece in the piecewise linear model of the targettile group/slice is set as follows: {0, 65, 66, 64, 67, 62, 62, 64, 64,64, 67, 64, 64, 62, 61, 0}. Then there are only 2 possible scalingfactor values for the entire tile group, because any codeword value from64 to 67 can have a same scaling factor value (e.g., 1.0 in fractionalprecision), and any codeword values from 60 to 63 can have the samescaling factor value (e.g., 60/64 = 0.9375 in factional precision). Forthe two end pieces having no codeword assigned to them, the chromascaling factor can be set to 1.0 by default. Therefore, in this example,one-bit is sufficient to signal lmcs_chroma_scaling_idx for blocks inthe target slice. The block can include a CU, a CTU, or a fixed areadepending on the chroma scaling factor signaling level.

Other than deriving the number of possible chroma scaling factor valuesusing the piecewise linear model, in some embodiments, the encoder cansignal a set of chroma scaling factor values at the tile group/sliceheader. Then, at the block level, the chroma scaling factor value can bedetermined using this set and the lmcs_chroma_scaling_idx value for thatblock.

Alternatively, to reduce the signaling cost, the chroma scaling factorcan be predicted from the neighboring blocks. For example, a flag can beused to indicate that the chroma scaling factor of the target block isequal to that of a neighboring block of the target block. Theneighboring block can be a top or left neighboring block. Thus, up to 2bits can be signaled for a target block. For example, in the 2 bits, afirst bit can indicate whether the chroma scaling factor of the targetblock is equal to that of a left neighbor of the target block, and asecond bit can indicate whether the chroma scaling factor of the targetblock is equal to a top neighbor of the target block. If the values ofneither bits indicate the chroma scaling factor of the target block isequal to the top neighbor or the left neighbor, then thelmcs_chroma_scaling_idx syntax can be signaled.

Dependent on the possibility of different values of“lmcs_chroma_scaling_idx,” variable length codewords can be used to“code lmcs_chroma_scaling_idx” to reduce the average code length.

Context-Based Adaptive Binary Arithmetic Coding (CABAC) can be appliedto code “lmcs_chroma_scaling_idx” of a target block. CABAC contextassociated with the target block can depend on “lmcs_chroma_scaling_idx”of neighboring blocks of the target block. For example, a leftneighboring block or a top neighboring block can be used to form theCABAC context. In terms of binarization of “lmcs_chroma_scaling_idx,”truncated Rice binarization can be used to binarize“lmcs_chroma_scaling_idx.”

By signaling the “lmcs_chroma_scaling_idx,” the encoder can select anadaptive lmcs_chroma_scaling_idx in terms of rate distortion cost.Accordingly, the lmcs_chroma_scaling_idx can be chosen using ratedistortion optimization to improve coding efficiency, which could helpto offset the signaling cost increase.

Embodiments of the disclosure further provide a method for processingvideo content with signaling a LMCS piecewise linear model.

Although the LMCS method in VVC draft 4 uses a piecewise linear modelwith 16 pieces, a number of unique values of SignaledCW[i] in a tilegroup/slice tends to be much less than 16. For example, some of the 16pieces can use a default number of codewords “OrgCW,” and some of the 16pieces can have a same number of codewords as each other. Therefore, insignaling the LMCS piecewise linear model, a number of unique codewordscan be signaled in a form of “listUniqueCW[],” and then, for each pieceof the LMCS piecewise linear model, an index of the listUniqueCW[] canbe sent for selecting a codeword for a target piece.

The modified syntax table is provided in Table 12 of FIG. 22 , in whichsyntax elements 2202 and 2204, shown in italics, are revised accordingto the present embodiment.

Semantics of the disclosed signaling method are as follows, with changesbeing underlined:

reshaper_model_min_bin_idx specifies the minimum bin (or piece) index tobe used in the reshaper construction process. The value ofreshape_model_min_bin_idx shall be in the range of 0 to MaxBinIdx,inclusive. The value of MaxBinIdx shall be equal to 15.

reshaper_model_delta_max_bin_idx specifies the maximum allowed bin (orpiece) index MaxBinIdx minus the maximum bin index to be used in thereshaper construction process. The value of reshape_model_max_bin_idx isset equal to MaxBinIdx

-   reshape_model_delta_max_bin_idx.

reshaper_model_bin_delta_abs_cw_prec_minus1 plus 1 specifies the numberof bits used for the representation of the syntax reshape_model_bin_delta_abs_CW[ i ].

reshaper_model_bin_num_unique_cw_minus1 plus 1 specifies the size of thecodeword array listUniqueCW.

reshaper_model_bin_delta_abs_CW[ i ] specifies the absolute deltacodeword value for the i-th bin.

reshaper_model_bin_delta_sign_CW_flag[ i ] specifies the sign ofreshape_model_bin_delta_abs_CW[ i ] as follows:

-   If reshape_model_bin_delta_sign_CW_flag[ i ] is equal to 0, the    corresponding variable RspDeltaCW[ i ] is a positive value.-   Otherwise ( reshape_model_bin_delta_sign_CW_flag[ i ] is not equal    to 0 ), the corresponding variable RspDeltaCW[ i ] is a negative    value.

When reshape_model_bin_delta_sign_CW_flag[ i ] is not present, it isinferred to be equal to 0.

The Variable RspDeltaCW[ I ] Is Derived as RspDeltaCW[ I ] = (1-2*Reshape_Model_Bin_Delta_Sign_CW [ I ]) *Reshape_Model_Bin_Delta_ABS_CW [ I ]

The variable listUniqueCW[ 0 ] is set equal to OrgCW. The variablelistUniqueCW[ i ] with i = 1 ... reshaper model bin num unique cwminus1, inclusive, is derived as follow:

-   The variable OrgCW is set equal to (1 << BitDepth_(Y) ) / (    MaxBinIdx + 1).-   listUniqueCW [ i ] = OrgCW + RspDeltaCW[ i 1 ]

reshaper model bin cw idx [ i ] specifies the index of the arraylistUniqueCW[] used to derive RspCW [ i ]. The value of reshaper modelbin cw idx [ i ] shall be in the range of 0 to (reshaper model bin numunique cw minus1 + 1), inclusive.

RspCW[ i ] is derived as follows:

-   If reshaper_model_min_bin_idx < = i <= reshaper_model_max_bin_idx    RspCW[ i ] = listUniqueCW[ reshaper_model_bin_cw_idx [ i ] ].-   Otherwise, RspCW[ i ] = 0.

The value of RspCW [ i ] can be in the range of 32 to 2 * OrgCW - 1 ifthe value of BitDepth_(Y) is equal to 10.

Embodiments of the disclosure provide a method for processing videocontent with conditional chroma scaling at a block level.

As shown in Table 1 of FIG. 6 ), whether chroma scaling is applied canbe determined by the tile_group_reshaper_chroma_residual_scale_flagsignaled at the tile group/slice level. However, it can be beneficial todetermine whether to apply chroma scaling at the block level. Forexample, in some embodiments, a CU level flag can be signaled toindicate if chroma scaling is applied to the target block. Presence ofthe CU level flag can be conditioned upon the tile group level flag“tile_group_reshaper_chroma_residual_scale_flag.” In other words, the CUlevel flag can be signaled only if chroma scaling is allowed at the tilegroup/slice level. While the CU level flag can allow the encoder tochoose whether to use chroma scaling based on whether the chroma scalingis beneficial for the target block, it can also incur signalingoverhead.

Consistent with the disclosed embodiments, to avoid the above signalingoverhead, whether chroma scaling is applied to a block can beconditioned upon a prediction mode of the target block . For example, ifthe target block is inter predicted, the prediction signal tends to begood, especially if its reference pictures are closer in temporaldistance. In this case, because the residual is expected to be verysmall, chroma scaling can be bypassed. For example, pictures in thehigher temporal levels tend to have reference pictures that are close intemporal distance, and for blocks in these pictures using nearbyreference pictures, chroma scaling can be disabled. The picture ordercount (POC) difference between the target picture and the target block’sreference pictures can be used to determine if this condition is met.

In some embodiments, chroma scaling can be disabled for all inter codedblocks. In some embodiments, chroma scaling can be disabled for allintra coded blocks. In some embodiments, chroma scaling can be disabledfor the combined intra/inter prediction (CIIP) mode, which is defined inthe VVC standard.

In the VVC standard, the CU syntax structure “coding_unit()” can includea syntax element “cu_cbf” to indicate if there is any non-zero residualcoefficient in the target CU. At the TU level, the TU syntax structure“transform_unit()” can include syntax elements “tu_cbf_cb” and“tu_cbf_cr” to indicate if there is any non-zero chroma (Cb or Cr)residual coefficients in the target TU. The chroma scaling process canbe conditioned upon these flags. As explained above, the averaging ofthe corresponding luma chroma scaling process can be invoked if there isno non-zero residual coefficient, then the chroma scaling process can bebypassed, and the present disclosure provides a method to bypass theluma averaging process.

Embodiments of the disclosure provide a method for processing videocontent with CCLM parameters derivation.

As discussed earlier, in the VVC 5, the parameters of CCLM forprediction of the target chroma block are derived with the luma andchroma reconstructed samples from neighboring blocks. In the case of adual tree, a luma block partition and a chroma block partitions can beunaligned. In other words, to derive the CCLM parameters for one N×Mchroma block, multiple neighboring luma blocks or a luma block with sizelarger than 2N×2M (in case of color format 4:2:0) can be reconstructed,thus incurring latency.

To reduce latency, as an example, the CCLM parameters are derived at theCTU/VPDU level. The reconstructed luma and chroma samples fromneighboring CTUs/VPDUs can be used to derive the CCLM parameters. Andderived parameters can be applied to all blocks within the CTU/VPDU. Forexample, formulas described in the cross-component linear modelprediction can be used to derive the parameters with X_(a) and Y_(a)being a luma value and a chroma value of the luma sample position withmaximum luma sample value among the CTU/VPDU neighboring luma samples,respectively. And X_(b) and Y_(b) represent the luma value and thechroma value of the luma sample position with minimum luma sample amongthe CTU/VPDU neighboring luma samples, respectively. To a person skilledin the art, any other derivation processes can be used in combinationwith the CTU/VPDU level parameter derivation concept proposed herein.

Besides CTU/VPDU level CCLM parameter derivation, such a derivationprocess can be performed on a fixed luma grid. In the VVC draft 5, whendual tree partitioning is used, separate luma and chroma partitions canstart from 64×64 luma grid. In other words, the split from 128×128 CTUto 64×64 CU can be performed jointly and not separately for luma andchroma. Therefore, as another example, CCLM parameters can be derived ona 64×64 luma grid. The neighboring reconstructed luma and chroma samplesof a 64×64 grid unit can be used to derive the CCLM parameters for allchroma blocks within the 64×64 grid unit. Compared to the CTU levelderivation, which can be up to 128×128 in luma samples, a 64×64 unitlevel derivation can be more accurate, and still does not have thepipeline latency issue as in the current VVC draft 5. On top of thisexample, CCLM parameters derivation can be further simplified byskipping derivation for some girds. For example, CCLM parameters areonly derived on the first 64×64 block within CTU and the derivation forfollowing 64×64 blocks in the same CTU is skipped. The parametersderived based on first 64×64 block can be used for all the blocks withinthe CTU.

FIG. 23 illustrates a flowchart of an exemplary method 2300 forprocessing video content, according to some embodiments of thedisclosure. In some embodiments, method 2300 can be performed by a codec(e.g., an encoder in FIGS. 2A-2B or a decoder in FIGS. 3A-3B). Forexample, the codec can be implemented as one or more software orhardware components of an apparatus (e.g., apparatus 400) for encodingor transcoding a video sequence. In some embodiments, the video sequencecan be an uncompressed video sequence (e.g., video sequence 202) or acompressed video sequence that is decoded (e.g., video stream 304). Insome embodiments, the video sequence can be a monitoring video sequence,which can be captured by a monitoring device (e.g., the video inputdevice in FIG. 4 ) associated with a processor (e.g., processor 402) ofthe apparatus. The video sequence can include multiple pictures. Theapparatus can perform method 2300 at the level of pictures. For example,the apparatus can process one picture at a time in method 2300. Foranother example, the apparatus can process a plurality of pictures at atime in method 2300. Method 2300 can include steps as below.

At step 2302, data representing a first block and a second block in apicture can be received. The plurality of blocks can include a firstblock and a second block. In some embodiments, the first block can be atarget chroma block (e.g., chroma block 1600 of FIG. 16A), and thesecond block can be a coding tree block (CTB), a transform unit (TU), ora virtual pipeline data unit (VPDU). The virtual pipeline data unit is anon-overlapping unit in the picture with a size that is less than orequal to a size of a coding tree unit for the picture. For example, whena size of a CTU is 128×128 pixels, a VPDU can have a smaller size thanthat of the CTU, and the size (e.g., 64×64 pixels) of the VPDU can beproportional to a buffer size in most pipeline stages of the hardware(e.g., hardware decoders).

In some embodiments, the coding tree block can be a luma block (e.g.,luma block 1610 of FIG. 16B) corresponding to the target chroma block.Accordingly, the data can include a plurality of chroma samplesassociated with the first block and a plurality of luma samplesassociated with the second block. The plurality of chroma samplesassociated with the first block comprises: a plurality of chromaresidual samples within the first block.

At step 2304, an average value of the plurality of luma samplesassociated with the second block can be determined. The plurality ofluma samples can include samples described with reference to FIGS. 18-19. As an example, as shown in FIG. 19 , The plurality of luma samples caninclude a plurality of reconstructed luma samples (e.g., a shaded sample1905 and a padded sample 1903) on a left boundary 1901 of the secondblock (e.g., 1902) or on a top boundary of the second block. It isappreciated that the plurality of reconstructed luma samples can belongto a neighboring reconstructed luma block (e.g., 1904).

Method 2300 can further include determining, among the plurality of lumasamples associated with the second block, whether a first luma sample isout of a boundary of the picture; and in responding to a determinationthat the first luma sample is out of the boundary of the picture,setting a value of the first luma sample to a value of a second lumasample of the plurality of luma samples that is within the boundary ofthe picture. The boundary of the picture can include one of a rightboundary of the picture and a bottom boundary of the picture. Forexample, it can be determined that padded sample 1903 is out of thepicture bottom boundary, and therefore, the value of padded sample 1903is set to be the value of shaded sample 1905 that is a sample closest tothe padded sample 1903 among all the samples on the picture bottomboundary.

It is appreciated that when the second block (e.g., 1902) is across aboundary of a picture, padded samples (e.g., padded sample 1903) can becreated, so that a number of the plurality of luma samples can be afixed number that usually is a power of 2 to avoid divisionaloperations.

At step 2306, a chroma scaling factor for the first block can bedetermined based on the average value. As discussed above with referenceto FIG. 18 , in intra prediction, decoded samples in a neighboring blockof the same picture can be used as reference samples to generate theprediction block. For example, the average value of the samples in aneighboring block can be used as a luma average value for determining achroma scaling factor of a target block (e.g., the first block in thisexample), and the chroma scaling factor for the first block can bedetermined using the luma average value of the second block.

At step 2308, the plurality of chroma samples associated with the firstblock can be processed using the chroma scaling factor. As discussedabove with reference to FIG. 5 , a plurality of chroma scaling factorscan construct a chroma scaling factor LUT at a tile group level, and beapplied at a decoder side on the reconstructed chroma residual of atarget block. Similarly, the chroma scaling factors can also be appliedat an encoder side.

In some embodiments, a non-transitory computer-readable storage mediumincluding instructions is also provided, and the instructions may beexecuted by a device (such as the disclosed encoder and decoder), forperforming the above-described methods. Common forms of non-transitorymedia include, for example, a floppy disk, a flexible disk, hard disk,solid state drive, magnetic tape, or any other magnetic data storagemedium, a CD-ROM, any other optical data storage medium, any physicalmedium with patterns of holes, a RAM, a PROM, and EPROM, a FLASH-EPROMor any other flash memory, NVRAM, a cache, a register, any other memorychip or cartridge, and networked versions of the same. The device mayinclude one or more processors (CPUs), an input/output interface, anetwork interface, and/or a memory.

The embodiments may further be described using the following clauses:

-   1. A computer-implemented method for processing video content, the    method comprising:    -   receiving data representing a first block and a second block in        a picture, the data comprising a plurality of chroma samples        associated with the first block and a plurality of luma samples        associated with the second block;    -   determining an average value of the plurality of luma samples        associated with the second block;    -   determining a chroma scaling factor for the first block based on        the average value; and    -   processing the plurality of chroma samples associated with the        first block using the chroma scaling factor.-   2. The method according to clause 1, wherein the plurality of luma    samples associated with the second block comprises:    -   a plurality of reconstructed luma samples on a left boundary of        the second block or on a top boundary of the second block.-   3. The method according to clause 2, further comprising:    -   determining, among the plurality of luma samples associated with        the second block, whether a first luma sample is out of a        boundary of the picture; and    -   in response to a determination that the first luma sample is out        of the boundary of the picture, setting a value of the first        luma sample to a value of a second luma sample of the plurality        of luma samples that is within the boundary of the picture.-   4. The method according to clause 3, further comprising:    -   determining, among the plurality of luma samples associated with        the second block, whether a first luma sample is out of a        boundary of the picture; and    -   in response to a determination that the first luma sample is out        of the boundary of the picture, setting a value of the first        luma sample to a value of a second luma sample of the plurality        of luma samples that is on the boundary of the picture.-   5. The method according to clause 4, wherein the boundary of the    picture is one of a right boundary of the picture and a bottom    boundary of the picture.-   6. The method according to any one of clauses 1-5, wherein the    second block is a coding tree block, a transform unit, or a virtual    pipeline data unit, wherein a size of the virtual pipeline data unit    is equal to or less than a size of a coding tree unit for the    picture.-   7. The method according to clause 6, wherein the virtual pipeline    data unit is a non-overlapping unit in the picture.-   8. The method according to any one of clauses 1-7, wherein the    plurality of chroma samples associated with the first block    comprises: a plurality of chroma residual samples within the first    block.-   9. The method according to any one of clauses 1-8, wherein the first    block is a target chroma block, and the second block is a luma block    corresponding to the target chroma block.-   10. A system for processing video content, comprising:    -   a memory for storing a set of instructions; and    -   at least one processor configured to execute the set of        instructions to cause the system to perform:        -   receiving data representing a first block and a second block            in a picture, the data comprising a plurality of chroma            samples associated with the first block and a plurality of            luma samples associated with the second block;        -   determining an average value of the plurality of luma            samples associated with the second block;        -   determining a chroma scaling factor for the first block            based on the average value; and        -   processing the plurality of chroma samples associated with            the first block using the chroma scaling factor.-   11. The system according to clause 10, wherein the plurality of luma    samples associated with the second block comprises:    -   a plurality of reconstructed luma samples on a left boundary of        the second block or on a top boundary of the second block.-   12. The system according to clause 11, wherein the at least one    processor is configured to execute the set of instructions to cause    the system to further perform:    -   determining, among the plurality of luma samples associated with        the second block, whether a first luma sample is out of a        boundary of the picture; and    -   in response to a determination that the first luma sample is out        of the boundary of the picture, setting a value of the first        luma sample to a value of a second luma sample of the plurality        of luma samples that is within the boundary of the picture.-   13. The system according to clause 12, wherein the second luma    sample is on the boundary of the picture.-   14. The system according to clause 13, wherein the boundary of the    picture is one of a right boundary of the picture and a bottom    boundary of the picture.-   15. The system according to any one of clauses 10-14, wherein the    second block is a coding tree block, a transform unit, or a virtual    pipeline data unit, wherein a size of the virtual pipeline data unit    is equal to or less than a size of a coding tree unit for the    picture.-   16. The system according to clause 15, wherein the virtual pipeline    data unit is a non-overlapping unit in the picture.-   17. The system according to any one of clauses 10-16, wherein the    plurality of chroma samples associated with the first block    comprises: a plurality of chroma residual samples within the first    block.-   18. The system according to any one of clauses 10-17, wherein the    first block is a target chroma block, and the second block is a luma    block corresponding to the target chroma block.-   19. A non-transitory computer readable medium that stores a set of    instructions that is executable by at least one processor of a    computer system to cause the computer system to perform a method for    processing video content, the method comprising:    -   receiving data representing a first block and a second block in        a picture, the data comprising a plurality of chroma samples        associated with the first block and a plurality of luma samples        associated with the second block;    -   determining an average value of the plurality of luma samples        associated with the second block;    -   determining a chroma scaling factor for the first block based on        the average value; and processing the plurality of chroma        samples associated with the first block using the chroma scaling        factor.-   20. The non-transitory computer readable medium according to clause    19, wherein the plurality of luma samples associated with the second    block comprises:    -   a plurality of reconstructed luma samples on a left boundary of        the second block or on a top boundary of the second block.

It should be noted that, the relational terms herein such as “first” and“second” are used only to differentiate an entity or operation fromanother entity or operation, and do not require or imply any actualrelationship or sequence between these entities or operations. Moreover,the words “comprising,” “having,” “containing,” and “including,” andother similar forms are intended to be equivalent in meaning and be openended in that an item or items following any one of these words is notmeant to be an exhaustive listing of such item or items, or meant to belimited to only the listed item or items.

As used herein, unless specifically stated otherwise, the term “or”encompasses all possible combinations, except where infeasible. Forexample, if it is stated that a database may include A or B, then,unless specifically stated otherwise or infeasible, the database mayinclude A, or B, or A and B. As a second example, if it is stated that adatabase may include A, B, or C, then, unless specifically statedotherwise or infeasible, the database may include A, or B, or C, or Aand B, or A and C, or B and C, or A and B and C.

It is appreciated that the above described embodiments can beimplemented by hardware, or software (program codes), or a combinationof hardware and software. If implemented by software, it may be storedin the above-described computer-readable media. The software, whenexecuted by the processor can perform the disclosed methods. Thecomputing units and other functional units described in this disclosurecan be implemented by hardware, or software, or a combination ofhardware and software. One of ordinary skill in the art will alsounderstand that multiple ones of the above described modules/units maybe combined as one module/unit, and each of the above describedmodules/units may be further divided into a plurality ofsub-modules/sub-units.

In the foregoing specification, embodiments have been described withreference to numerous specific details that can vary from implementationto implementation. Certain adaptations and modifications of thedescribed embodiments can be made. Other embodiments can be apparent tothose skilled in the art from consideration of the specification andpractice of the invention disclosed herein. It is intended that thespecification and examples be considered as exemplary only, with a truescope and spirit of the invention being indicated by the followingclaims. It is also intended that the sequence of steps shown in figuresare only for illustrative purposes and are not intended to be limited toany particular sequence of steps. As such, those skilled in the art canappreciate that these steps can be performed in a different order whileimplementing the same method.

In the drawings and specification, there have been disclosed exemplaryembodiments. However, many variations and modifications can be made tothese embodiments. Accordingly, although specific terms are employed,they are used in a generic and descriptive sense only and not forpurposes of limitation.

What is claimed is:
 1. A method of decoding a bitstream to output one ormore pictures for a video stream, the method comprising: determining,based on encoded syntax elements in the bitstream, a chroma scalingfactor associated with a target coding block; and performing chromaresidual scaling of the target coding block using the chroma scalingfactor, wherein the chroma scaling factor is determined based on N lumasamples, the N being independent of a size of the target coding block,and wherein when one or more of the N luma samples are outside aboundary of a picture, the one or more of the N luma samples arereplaced with a luma sample on the picture boundary.
 2. The method ofclaim 1, wherein the encoded syntax elements include an index signalingthe chroma scaling factor or being used for deriving the chroma scalingfactor.
 3. The method of claim 1, wherein the luma sample on the pictureboundary: is determined from a plurality of luma samples on the pictureboundary, and has a distance closest to the one or more of the N lumasamples outside the picture boundary.
 4. The method of claim 1, whereinthe picture boundary is one of a right boundary or bottom boundary ofthe picture.
 5. The method of claim 1, wherein N is a power of
 2. 6. Themethod of claim 1, wherein N is a fixed number.
 7. The method of claim1, wherein the N is dependent on a size of a coding tree block (CTB). 8.The method of claim 1, wherein the N is less than or equal to the sizeof the coding tree unit for the picture.
 9. The method of claim 1,wherein the chroma scaling factor is determined based on an averagevalue of the N luma samples.
 10. A video decoder, comprising: a memoryfigured to store instructions; and one or more processors configured toexecute the instructions to cause the video decoder to perform:determining, based on encoded syntax elements in a bitstream associatedwith a video sequence, a chroma scaling factor associated with a targetcoding block; and performing chroma residual scaling of the targetcoding block using the chroma scaling factor, wherein the chroma scalingfactor is determined based on N luma samples, the N being independent ofa size of the target coding block, and wherein when one or more of the Nluma samples are outside a boundary of a picture, the one or more of theN luma samples are replaced with a luma sample on the picture boundary.11. The video decoder of claim 10, wherein the encoded syntax elementsinclude an index signaling the chroma scaling factor or being used forderiving the chroma scaling factor.
 12. The video decoder of claim 10,wherein the luma sample on the picture boundary: is determined from aplurality of luma samples on the picture boundary, and has a distanceclosest to the one or more of the N luma samples outside the pictureboundary.
 13. The video decoder of claim 10, wherein the pictureboundary is one of a right boundary or bottom boundary of the picture.14. The video decoder of claim 10, wherein N is a power of
 2. 15. Thevideo decoder of claim 10, wherein N is a fixed number.
 16. The videodecoder of claim 10, wherein the N is dependent on a size of a codingtree block (CTB).
 17. The video decoder of claim 10, wherein the N isless than or equal to the size of the coding tree unit for the picture.18. The video decoder of claim 10, wherein the chroma scaling factor isdetermined based on an average value of the N luma samples.
 19. A methodof encoding a video sequence into a bitstream, the method comprising:determining, based on N luma samples, a chroma scaling factor associatedwith a target coding block, the N being independent of a size of thetarget coding block; and encoding, in the bitstream, one or more syntaxelements signaling the chroma scaling factor, wherein when one or moreof the N luma samples are outside a boundary of a picture, the one ormore of the N luma samples are replaced with a luma sample on thepicture boundary.
 20. The method of claim 19, wherein the one or moresyntax elements include an index signaling the chroma scaling factor orbeing used for deriving the chroma scaling factor.